mips: ralink: enable zboot support

Some of these ralink devices come with an ancient u-boot which can't
extract LZMA properly when image gets too big.
Enable zboot support to get a self-extracting kernel instead of relying
on broken u-boot support.

Signed-off-by: Chuanhong Guo <gch981213@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>