unmkinitramfs: Rewrite in C to make it acceptably fast

Parsing uncompressed cpio archives in this shell script requires running multiple processes for each archive member.

This was not too bad when there were usually only a few microcode blobs in an uncompressed archive, but now that we put compressed kernel modules in an uncompressed archive, it became very slow indeed.

Rewrite it in C so that it's acceptably fast.

This should have no functional changes, except that it adds support for an initramfs with no compressed part.

Fixes: 81fd41f7 ("Put compressed kernel modules and firmware in an uncompressed cpio")
Signed-off-by: Ben Hutchings benh@debian.org