ar71xx: add profile and build image for the EnGenius ESR900

Signed-off-by: Forest Crossman <cyrozap@gmail.com>
Patchwork: http://patchwork.openwrt.org/patch/5715/


[juhosg: change subject]
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

SVN-Revision: 41642