rb532: fix initramfs image creation (#14215)

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38219 3c298f89-4303-0410-b956-a3cf2f4a3e73
opkg: add build variant with signature support

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38220 3c298f89-4303-0410-b956-a3cf2f4a3e73