ramips: add support for Beeline SmartBox GIGA

ramips: add support for Beeline SmartBox GIGA

Beeline SmartBox GIGA is a wireless WiFi 5 router manufactured by
Sercomm company.

Device specification
--------------------
SoC Type: MediaTek MT7621AT
RAM: 256 MiB, Nanya NT5CC128M16JR-EK
Flash: 128 MiB, Macronix MX30LF1G18AC
Wireless 2.4 GHz (MT7603EN): b/g/n, 2x2
Wireless 5 GHz (MT7613BE): a/n/ac, 2x2
Ethernet: 3 ports - 2xGbE (WAN, LAN1), 1xFE (LAN2)
USB ports: 1xUSB3.0
Button: 1 button (Reset/WPS)
PCB ID: DBE00B-1.6MM
LEDs: 1 RGB LED
Power: 12 VDC, 1.5 A
Connector type: barrel
Bootloader: U-Boot

Installation
-----------------
1. Downgrade stock (Beeline) firmware to v.1.0.02;
2. Give factory OpenWrt image a shorter name, e.g. 1001.img;
3. Upload and update the firmware via the original web interface.

Remark: You might need make the 3rd step twice if your running firmware
is booted from the Slot 1 (Sercomm0 bootflag). The stock firmware
reverses the bootflag (Sercomm0 / Sercomm1) on each firmware update.

Revert to stock
---------------
1. Change the bootflag to Sercomm1 in OpenWrt CLI and then reboot:
      printf 1 | dd bs=1 seek=7 count=1 of=/dev/mtdblock3
2. Optional: Update with any stock (Beeline) firmware if you want to
   overwrite OpenWrt in Slot 0 completely.

MAC Addresses
-------------
+-----+-----------+---------+
| use | address   | example |
+-----+-----------+---------+
| LAN | label     | *:16    |
| WAN | label + 1 | *:17    |
| 2g  | label + 4 | *:1a    |
| 5g  | label + 5 | *:1b    |
+-----+-----------+---------+
The label MAC address was found in Factory 0x21000

Notes
-----
1. The following scripts are required for the build:
      sercomm-crypto.py - already exists in OpenWrt
      sercomm-partition-tag.py - already exists in OpenWrt
      sercomm-payload.py - already exists in OpenWrt
      sercomm-pid.py - new, the part of this pull request
      sercomm-kernel-header.py - new, the part of this pull request
2. This device (same as other Sercomm S2,S3-based devices) requires
   special LZMA and LOADADDR settings for successful boot:
      LZMA_TEXT_START=0x82800000
      KERNEL_LOADADDR=0x81001000
      LOADADDR=0x80001000
3. This device (same as several other Sercomm-based devices - Beeline,
   Netgear, Etisalat, Rostelecom) has partition map (mtd1) containing
   real partition offsets, which may differ from device to device
   depending on the number and location of bad blocks on NAND.
   "fixed-partitions" is used if the partition map is not found or
   corrupted. This behavour (it's the same as on stock firmware) is
   provided by MTD_SERCOMM_PARTS module.

Signed-off-by: Mikhail Zhilkin <csharper2005@gmail.com>