target/imagebuilder: remove obsolete !TARGET_ROOTFS_INITRAMFS dependency

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43128 3c298f89-4303-0410-b956-a3cf2f4a3e73