ar71xx: add dummy rle_decode function if CONFIG_RLE_DECOMPRESS is not set

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39079 3c298f89-4303-0410-b956-a3cf2f4a3e73