include/image.mk: do not make initramfs build exclusive

Allow building initramfs along with other types of filesystems.

Signed-off-by: Florian Fainelli <florian@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@37050 3c298f89-4303-0410-b956-a3cf2f4a3e73