mediatek: MERCUSYS MR90X v1: add OpenWrt U-Boot (UBI) layout

mediatek: MERCUSYS MR90X v1: add OpenWrt U-Boot (UBI) layout

This commit adds OpenWrt U-Boot (UBI) layout support for MERCUSYS MR90X
v1.

Stock U-Boot UBI size:   50 MiB
OpenWrt U-boot UBI size: 126 MiB

Install
-------
1. Perform steps 1-14 of the OpenWrt installation guide (use OpenWrt
   initramfs-recovery.itb instead of initramfs-kernel.bin at the step 10,
   12 and 14).
Link: https://openwrt.org/toh/mercusys/mr90x_v1#installation
2. Make backups:
```
cat /dev/mtd0 > /tmp/boot.bin
cat /dev/mtd5 > /tmp/tp_data.bin
```
   Copy /tp_data dir content, /tmp/boot.bin and /tmp/tp_data.bin and to
   your PC using scp. You can also backup the remaining partititons. Copy
   backups to a safe place, they are required for the next steps and stock
   firmware recovery.

3. Reboot to OpenWrt initramfs:
```
reboot
```

4. Copy OpenWrt ubi-bl31-uboot.fip, ubi-preloader.bin,
   ubi-squashfs-sysupgrade.itb and MT7986_EEPROM.bin, default-mac (from
   /tp_data backup) to the /tmp folder of the router using scp.

5. Prepare UBI:
```
ubidetach -p /dev/mtd3; ubiformat /dev/mtd3 -y; ubiattach -p /dev/mtd3
ubimkvol /dev/ubi0 -N fip -t static -s 1MiB
ubiupdatevol /dev/ubi0_0 /tmp/ubi-bl31-uboot.fip
ubimkvol /dev/ubi0 -N ubootenv -s 0x1f000
ubimkvol /dev/ubi0 -N ubootenv2 -s 0x1f000
```

6.  Install kmod-mtd-rw and unlock partitions:
```
opkg update && opkg install kmod-mtd-rw
insmod mtd-rw i_want_a_brick=1
mtd unlock boot
mtd unlock bl2
mtd unlock factory
```

7. Prepare "factory" partition:
```
dd if=/dev/zero bs=$((0x8000)) count=1 | tr '\000' '\377' > /tmp/factory.bin
dd if=/tmp/MT7986_EEPROM.bin of=/tmp/factory.bin conv=notrunc
dd if=/tmp/default-mac >> /tmp/factory.bin
```

8. Write "factory" partition:
```
mtd erase factory
mtd write /tmp/factory.bin factory
```

9. Write preloader partition:
```
mtd erase bl2
mtd write /tmp/ubi-preloader.bin bl2
```

10. Write OpenWrt sysupgrade image:
```
sysupgrade -n /tmp/ubi-squashfs-sysupgrade.itb
```

Recovery
--------
1. Place OpenWrt initramfs-recovery.itb image (with original name) on the
   tftp server (IP: 192.168.1.254).
2. Press "reset" button and power on the router. After ~10 sec release the
   button.
3. Use OpenWrt initramfs system for recovery.

BL2 and FIP recovery
--------------------
Use mtk_uartboot and UART connetion if BL2 or FIP in UBI is destroyed:
Link: https://github.com/981213/mtk_uartboot
Link: https://openwrt.org/toh/mercusys/mr90x_v1#serial
Return to stock:
----------------
1. Copy "boot" partition backup (boot.bin) to the /tmp dir of the router
   using scp.

2. Install kmod-mtd-rw:
```
opkg update && opkg install kmod-mtd-rw
```

3. Restore stock U-Boot:
```
   insmod mtd-rw i_want_a_brick=1
   mtd unlock boot
   mtd erase boot
   mtd write /tmp/boot.bin boot
```

4. Erase UBI and reboot:
```
mtd erase ubi
reboot
```

5. Open U-Boot web recovery, upload stock firmware image and start
   upgrade.
Link: http://192.168.1.1
6. Complete steps 1-9 of the OpenWrt installation guide to get root
   rights.
Link: https://openwrt.org/toh/mercusys/mr90x_v1#installation
7. Upload "tp_data" partition backup (tp_data.bin) to the /tmp folder of
   the router using scp.

8. Restore stock calibrations:
```
mtd write /tmp/tp_data.bin tp_data
reboot
```

9. Perform "factory restore" via stock firmware web interface.

Signed-off-by: Mikhail Zhilkin <csharper2005@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/16744
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>