please add api to repack initramfs

please add api to repack initramfs

from the results of unpackinitramfs, including all early & main dirs.

Userstory: i want to tweak my initrd in-place by replacing broken libc.so.6 with a patched one, I want to use unmkinitramfs, modify files in main, repack it and boot it.

CLI:

$ unmkinitramfs /boot/initrd.img unpacked-initrd
$ ls unpacked-initrd/
early early2 main
$ (edit things in unpacked-initrd/main/)
$ remkinitramfs unpacked-initrd patched-initrd.img

Where patched-initrd.img is now similar to /boot/initrd.img, apart from the changed things.

I expect remkinitramfs like functionality that will take the output of unmkinitramfs (e.g. unpacked-initrd/) directory as input, do uncompressed cpio archives out of all the subdirs, make compressed cpio archive out of main, and produce a patched-initrd.img that is ready for booting.

It is very close to what mkinitramfs does, but skip calling all the hooks to create things a fresh, and just do the final assembly step of cpio and compression.