scripts/gen_initramfs_list.sh: Convert to a /bin/sh script

Replace bashisms with POSIX-compatible shell scripting.

Notably, de-duplicate '/' using a sed command from elsewhere in the same script
rather than "${name//\/\///}".

Commit by Jamey Sharp and Josh Triplett.

Signed-off-by: Jamey Sharp <jamey@minilop.net>
Signed-off-by: Josh Triplett <josh@joshtriplett.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>