Draft: main/postmarketos-mkinitfs-hook-kernelmodules: new aport

Allow stuffing kernel modules into a boot.img and installing them on the fly. This is intended for local kernel development or in CI where it's desirable to test a kernel just by flashing or booting a boot.img file without having to install the new modules into the rootfs.

This requires running a hook after the rootfs has been mounted, we can do this by putting the scripts for this hook in /usr/share/mkinitfs/hooks/sysroot to avoid needlessly creating new /hook-* directories and adding them to mkinitfs.

TODO:

• Fix mkinitfs so that it properly handles subdirectories for hook directories https://gitlab.com/postmarketOS/postmarketos-mkinitfs/-/merge_requests/35
• Document the sysroot hooks on the wiki