ar71xx: fix UAP-PRO images

- it should be BOARDNAME instead of BOARD_NAME
- kernel partition should be padded to 1536k (somehow this padding was lost in translation to BuildCode in bc797c73)

Signed-off-by: Roman Yeryomin <roman@advem.lv>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46193 3c298f89-4303-0410-b956-a3cf2f4a3e73