Otherwise the root device won't be found in 1/16 of the generated images
(whenever the signature starts with a zero digit).

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@44959 3c298f89-4303-0410-b956-a3cf2f4a3e73