kbuild: fix unportability in gen_initramfs_list.sh

On a Mac OS X machine the output of ls -l is different from a standard
Linux machine. Use readlink instead of parsing a hardcoded field number
from the ls output.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>