image.mk: store the device name variable

Make the device name accessible for any build commands instead of
forcing targets to define it themselves.

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46597 3c298f89-4303-0410-b956-a3cf2f4a3e73