ramips: add profile and build image for the Tenda 3G300M

Factory image should be used to flash from
original firmware.

[juhosg: use the GENERIC_4M template]

Signed-off-by: Cezary Jackiewicz <cezary.jackiewicz@gmail.com>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

SVN-Revision: 35246