Add additional files into initramfs when debug-shell is installed

This PR uses the same approach as the Alpine's mkinitfs (also discussed in https://github.com/postmarketOS/pmbootstrap/issues/1188#issuecomment-363897670) to add the possibility to the hooks to include additional binaries to the initramfs.

The mkinitfs script search the *.files in the path /etc/postmarketos-mkinitfs/files/ and each line of those files is a binary file that has to be added to the initramfs.

Right now only the debug-shell hook uses this approach to add some files mentioned in #785 that are useful for the kernel mainlining process.