archiso/mkarchiso:
Introduce a buildmodes array, that can be used to build towards more than one output artifact type.
Add a buildmode for building a bootstrap image (a compressed file containing a very minimal Arch installation).
The buildmodes can be set either using a `buildmodes` array in a `profiledef.sh` or by using the `-m` option flag to
mkarchiso and providing a space delimited, quoted list.
The 'iso' buildmode is always the default if no buildmodes are setup.
Implement building a bootstrap image, when using the 'bootstrap' `buildmode`, which uses a profile's
'bootstrap_packages.$arch' file to install packages using pacstrap and compressing it to a bootstrap image.
The name of the output file is currently constructed from the `iso_name` value by appending `-bootstrap`.

Replace the uses of `airootfs_dir` with the more generic `pacstrap_dir`, as the location denotes where pacstrap is
being used.
Replace uses of `img_name` with `image_name` and removing it from the global scope, so that it can be overridden per
each buildmode.
Rename `_cleanup_airootfs_dir()` to `_cleanup_pacstrap_dir()`.
Make `_run_once()` more generic by prepending the state files with a string defined by `run_once_mode`.
Add `_validate_requirements_buildmode_all()`, `_validate_requirements_buildmode_bootstrap()` and
`_validate_requirements_buildmode_iso()` to validate the general requirements of the different buildmodes.
Add `_build_bootstrap_image()` to generate the bootstrap image using bsdtar.
Rename `_build_iso()` to `_build_iso_image()` to fit the naming of the respective bootstrap function.
Extend `_read_profile()` to include the reading of bootstrap image specific packages from a file.
Extend `_validate_options()` to include testing of the bootstrap packages and running of validation functions for all
buildmodes.
Change `_set_overrides()` to override the buildmodes if they are specified via the `-m` option flag.
Change `_make_version()` to be used generically in all buildmodes.
Change `_make_pkglist()` to be used generically in all buildmodes.
Rename `_build_profile()` to `_build_buildmode_iso()` and set local variables that are specific to the buildmode, such
as `image_name`, `pacstrap_dir`, `run_once_mode` , `buildmode_packages` and `buildmode_pkg_list`.
Add `_build_buildmode_bootstrap()` and set local variables that are specific to the buildmode, such as `image_name`,
`pacstrap_dir`, `run_once_mode` , `buildmode_packages` and `buildmode_pkg_list`.
Add the `-m` option flag to the list of flags.

Closes #127