do not hardcode binaries/files to include in initfs

The list of files to include in the initfs shouldn't be built into the binary, so that they can be changed without having to rebuild mkinitfs.

This will also make mkinitfs one step closer to being distro-agnostic

TODO:

• 1) split init.sh stuff from postmarketos-mkinitfs into postmarketos-initramfs-base
• 2) add a files hook to pmos-initramfs-base that installs the list to /etc/postmarketos-mkinitfs/files/00-base.files
• 3) figure out how to handle osk-sdl... and conditionally installing mesa. new way to handle the files hook?
• 4) remove hardcoded file lists from main.go
• 5) bump postmarketos-mkinitfs and do new release/package upgrade