kbuild: add support for squashing uid/gid in gen_initramfs_list.sh

Sometimes it is useful to squash all uid's/gid's to 0:0 regardless of
current owner.  For example, in build systems that get run as arbitrary
users (uClinux-dist).  This adds a special "squash" keyword so you can do
'-g squash -u squash' and have ownership squashed to root.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>