ramips: important patch for HG255D

ramips: important patch for HG255D

r37505 add support for Huawei HG255D, but it only tested under a community hacked u-boot ("lintel u-boot"), which has a different mtd layout compared to origin one. If you install it on a box with origin u-boot, the origin factory part will be destroyed, and your wifi interface will never up!!!

This patch shrink firmware part in mtd layout to exclude origin factory part, and  I will prepare another patch to fix the eeprom extract issue for box with origin u-boot.

Signed-off-by: Leon Xu <ylxu72@gmail.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@37530 3c298f89-4303-0410-b956-a3cf2f4a3e73