ar71xx: image: use a common macro for Buffalo WZRHP devices

The Image/Build/WZRHP64K and the Image/Build/WZRHP128K macros
are similar. Move the common stuff into the Image/Build/WZRHP
macro and update the orignal macros to use that.

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

SVN-Revision: 38152