ar71xx: optimize kernel generation

When the squashfs and jffs2 images are generated
in the same turn, the compressed kernel images
are generated twice with the same parameters.

Move compressed kernel generation into a separate
phase to avoid that.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@37881 3c298f89-4303-0410-b956-a3cf2f4a3e73