Various bash script improvements.

Replace build.sh scripts with calls to mkarchiso -B "profiledir" build_profile.

Fixes #37 .

There is nothing useful in /boot for the live system.

Boot mode names are:

- bios_syslinux.mbr: SYSLINUX in MBR
- bios.syslinux.eltorito: SYSLINUX (ISOLINUX) via El Torito
- uefi-x64.systemd-boot.esp: systemd-boot on ESP in MBR
- uefi-x64.systemd-boot.eltorito: systemd-boot on ESP via El Torito

It is not yet possible to create an ISO with only El Torito or only MBR boot modes!

Adapt _make_* functions to mkarchiso.

Related to #37 .

Nothing is implemented yet!

configs/releng/profiledef.sh:
A test profile.

Quote all variables.
Terminate option processing using '--' for commands that support it.
Do not hardcode file descriptor.
Compare integers with arithmetic comparison instead of string comparison.
Replace echo with printf.
Use heredoc for usage text.
Don't print INFO messages when quiet is set.
Export SOURCE_DATE_EPOCH.

exfatprogs uses the kernel's driver unlike exfat-utils which provides a FUSE driver.

An ISO's checksum and GPG validation primarily matters when PXE booting, so it would be appropriate to enable these options for releng.
They should protect against booting a corrupt image.

Apparently blockdev does not support it.
In an ISO made using '-s img' (Squashfs with dm-snapshot), it results in:

    blockdev: Unknown command: --

archiso/mkarchiso:
Add the -rational-rock option to xorriso.
This is a preparatory step for creating ISOs as a regular user. See archlinux/archiso#40 .

Fixes #35 .

Properly track the file instead of modifying it on-the-fly with customize_airootfs.sh.
See archlinux/archiso#21 .

`make check` could get extended to include other things besides just linting.

Makefile:
Move shellcheck to a "lint" target and change "check" so that it runs "lint".
This provides a more finer grained control and allows to extend "check" with things beyond just linting.

.gitlab-ci.yml:
Run `make lint` in the lint job.

configs/releng/airootfs/etc/systemd/system/reflector.service:
Use the 70 mirrors synced most recently (--latest) instead of a specific time since last sync (--age).
According to FS#67399 using 1hour as sync age is too short, as it sometimes leads to empty list.

They are not required in the host system. See #34 .

Place custom mkinitcpio.conf in airootfs.
Use a custom mkinitcpio preset to specify generated image file path.

configs/releng/airootfs/etc/fstab:
/etc/fstab shipped by the filesystem package has only comments. There is no reason to replace it with an empty file.

configs/releng/airootfs/etc/systemd/system/default.target:
There is no harm in booting to graphical.target. releng does not enable (or even install) any service that has {Required,Wanted}By=graphical.target.

Gets rid of a gpg warning:
gpg: WARNING: unsafe permissions on homedir '/tmp/mkinitcpio.*/root/gpg'

LICENSE:
Add GPL-3.0 license.

{{archiso,configs}/*,.editorconfig,.gitlab-ci.yml}:
Add SPDX license identifier.

Makefile:
Add SPDX license identifier.
Install the `run_archiso.sh` script as global executable `run_archiso`.
Use -D and -t flags to install to install files more generically (without a previous call to install the directory).

README.rst:
Add README outlining the project's scope, how to build images from the profiles and how to test.

AUTHORS.rst:
Add list of all direct contributors to the repository.

CONTRIBUTING.rst:
Add basic contribution guidelines, explaining the linter and the license in use.

Closes #7
Closes #3

configs/*:
Copy all files that do not need a rename generically (not specifying a destination file name).
Do not rename vmlinuz-linux to vmlinuz or vmlinuz.efi (as this serves no purpose and makes the scripts more
complicated).
Do not rename microcode (i.e. {amd,intel}-ucode.img) when copying them and change all boot loader configuration files
that assume a renamed microcode image.
Add note and link to Arch Linux wiki to state why memtest.bin is renamed to memtest.
Copy license files for {amd,intel}-ucode and memtest more generically by placing them into subdirectories with the same
name as the package (to circumenvent overwriting one other).

Closes #33

Additionally copy the files to all custom user homes, not just root's.

Show a deprecation notice if airootfs/root/customize_airootfs.sh is found.

configs/{baseline,releng}/build.sh:
Copy custom files to airootfs before installing packages.
Instead of calling `mkarchiso init`, list all required packages in packages.x86_64 and install them all at once with `mkarchiso install`. The mkdir command which `mkarchiso init` performs is now done by make_custom_airootfs.

configs/releng/build.sh:
Don't copy configs/releng/pacman.conf to airootfs, it is only meant to provide a unmodified pacman.conf durring pacstrap. In airootfs, an unmodified /etc/pacman.conf will be installed with the pacman package.

Install linux and mkinitcpio packages.
Remove root user's password.

Fixes https://bugs.archlinux.org/task/64236 .

Correct shadow file permissions from build.sh.

This reverts commit 2993b808.

Actions are disabled in archiso's GitHub mirror.

Having a shellcheck directive and custom comments in the same line can trigger SC1107 on old versions of shellcheck.

.github/workflows/lint.yml:
Run `make check`.

Makefile:
Move shellcheck commands from .gitlab-ci.yml.

.gitlab-ci.yml:
Run `make check`.
Prevent partial upgrades, i.e. use '-Syu'.
Prevent reinstalling already installed packages by using pacman's '--needed' option.
Don't force-download the package database.

When one calls the `build.sh` scripts with bash instead of the shebang
`$script_path` contains the filepath, not the parent dir:

```
% cd /path
% grep -A2 script_path= build.sh
script_path=$(readlink -f "${0%/*}")
echo "$script_path"
exit 0
% ./build.sh
/path
% bash build.sh
/path/build.sh
```

This commit fixes that:

```
% grep -A2 script_path= build.sh
script_path="$( cd -P "$( dirname "$(readlink -f "$0")" )" && pwd )"
echo "$script_path"
exit 0
% ./build.sh
/path
% bash build.sh
/path
```

linux only gets pulled in because of broadcom-wl and mkinitcpio (usually) gets pulled in by linux.

Move all required packages to packages.x86_64.
Test file availability before copying them.

archiso/initcpio/install/*:
Setting bash shebang for all scripts and making them comform with shellcheck.

archiso/initcpio/{hooks,script}/*:
Setting ash shebang for all scripts and making them comform with shellcheck (for dash, as shellcheck has no ash specific
ruleset). Essentially the ash based scripts should be POSIX compliant as much as possible to have an easier time
writing, debugging and maintaining them.
Ensuring that variables are not treated as options and introducing variable quoting.

.gitlab-ci.yml:
Integrating shellcheck for initcpio scripts.

Closes #32

/usr/local/bin/Installation_guide:
Add a convenience script which opens the installation guide in lynx.

/etc/motd:
Provide a message with minimal instructions for connecting to internet and inform about the Installation_guide convenience script.

dialog was only added because it was needed for netctl's wifi-menu.
Now that netctl is removed, nothing requires dialog.