Add support for MitraStar HGW-2501GN-R2

Hello, I’d like to request support for adding the OpenWrt firmware to the router MitraStar HGW-2501GN-R2.

It is a router made by MitraStar Technologies for the Spanish ISP Movistar (Telefonica).

It has a good hardware, these are the specifications:

`CPU: Ralink RT63368F (in motherboard) / Ralink RT63365 (for software)

Switch: Realtek RTL8368MB

Flash Storage: 64 MB

RAM: 128 MB

Wireless Chipset (Wl0): Ralink RT5392`

For more information, there is a link with more information (in Spanish):

In the link provided above, it provides some information of reverse engineering the firmware for installing software/firmware that is not designed/made by the ISP. However, with some later firmware updates, Movistar (the ISP) has patched some "bugs" like accessing to the file system with root permissions through SSH.

Other probably useful information is that the router every time reboots, it reverts any not saved change with the SquashFS, and for the firmware upgrades, it checks on the firmware file some heading (probably checksums).

An important note is that it must support the triple VLAN, because of the VoIP service and the IPTV (Movistar+) service.