image.mk: add Build step pad-to

pad-to can be used in a pipe to pad the image to a specific chunk-size.
Signed-off-by: Alexander Couzens <lynxis@fe80.eu>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@45980 3c298f89-4303-0410-b956-a3cf2f4a3e73