Add configs/*/grub/loopback.cfg

configs/*/grub/grub.cfg: rearrange module loading

• Do not manually load modules that will get loaded by invoking a command.
• Explicitly load serial modules.
• Move insmod all_video after the font is loaded.

configs/releng/grub/grub.cfg: play init tune at the end

Move the play command to the end of the file so it plays after the menu entries are shown and ready.

configs/*/grub/grub.cfg: do not hardcode the architecture and platform

Construct a human readable platform identifier from GRUB's built-in variables and use it in menu item descriptions.

Only add the menu entries for the additional tools (UEFI shell, Memtest86+) if the files exist.

Modify baseline's grub.cfg to closer match releng.

Add configs/*/grub/loopback.cfg

See https://www.supergrubdisk.org/wiki/Loopback.cfg for details.

Only ${iso_path} is guaranteed, so we need to search for the volume, on which the ISO file resides, ourselves.

Implements #165 (closed)

mkarchiso: always create /boot/grub/grubenv and /boot/grub/loopback.cfg

Even if GRUB is not used as a boot loader for the ISO, create a /boot/grub/grubenv file in the ISO 9660 file system. If a loopback.cfg file exists in the profile's grub directory, copy it to /boot/grub/loopback.cfg on the ISO.

This ensures the funtionality will not be lost if the used boot loaders are changed.