ar71xx: image: use a common macro for Cameo AP94 based devices

The Image/Build/DIR825B1 and the Image/Build/DLRTDEV macros
are identical. Merge those into a new Image/Build/CameoAP94
macro, and update the device profiles to use the new macro.
Also rename the dir825b1_* mtdlayout macros to cameo_ap94_*.

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

SVN-Revision: 38150