In r44391 the kernel partion size was increased to allow larger kernels,
but the rootfs partition offset was missed. Fix this by setting the
rootfs offset to the expected value.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@45868 3c298f89-4303-0410-b956-a3cf2f4a3e73