Do not output faulty ".ko" dependency if a scanned .ko file has an empty
"depends=" specification, also replace the `grep | sed | awk` chain with a
single sed program.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@44206 3c298f89-4303-0410-b956-a3cf2f4a3e73