gen_init_cpio: support file checksum archiving

Documentation/driver-api/early-userspace/buffer-format.rst includes the
specification for checksum-enabled cpio archives.  Implement support for
this format in gen_init_cpio via a new '-c' parameter.

Link: https://lkml.kernel.org/r/20220404093429.27570-6-ddiss@suse.de


Signed-off-by: David Disseldorp <ddiss@suse.de>
Suggested-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <christian.brauner@ubuntu.com>
Cc: Martin Wilck <mwilck@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>