mediatek: comfast cf-wr632ax: add all-in-UBI layout

mediatek: comfast cf-wr632ax: add all-in-UBI layout

This commit introduces OpenWrt U-Boot UBI layout support
for the COMFAST CF-WR632AX, enabling:
- Prolonged device lifetime by allocating most of the flash
  to UBI (which takes care of wear-leveling)
- Maximum available storage space for OpenWrt
- Fully-featured U-Boot
- Effective recovery mechanisms

OpenWrt U-Boot UBI flash instructions
-------------------------------------

A device running stock firmware should be upgraded to the
latest standard OpenWrt firmware
(https://firmware-selector.openwrt.org/?target=mediatek%2Ffilogic&id=comfast_cf-wr632ax).

Back up critical data
---------------------

Console:
1. While the device is running OpenWrt with stock MTD partitions:
   mtd dump BL2 > /tmp/BL2.bin
   mtd dump u-boot-env > /tmp/u-boot-env.bin
   mtd dump Factory > /tmp/Factory.bin
   mtd dump FIP > /tmp/FIP.bin
2. Copy the backup files to your PC via SCP.

LuCI Web-UI:
"System" -> "Backup / Flash Firmware" -> "Save mtdblock contents"
Save mtdblock:
   BL2
   u-boot-env
   Factory
   FIP
Make sure the files were successfully downloaded to your downloads directory,
especially the Factory file.

Using the installer image
-------------------------
To simplify the installation process, this method uses a fork
of Daniel Golle's (@dangowrt) UBI Installer
https://github.com/dangowrt/owrt-ubi-installer

1. Ensure your router is running the latest generic OpenWrt firmware.
   Upgrade it if necessary.
2. Obtain the installer image:
   Build the installer from source
   https://github.com/andros-ua/owrt-ubi-installer/tree/cf-wr632ax
   or download a prebuilt image from the
   https://github.com/andros-ua/owrt-ubi-installer/releases
3. Flash the openwrt*-ubi-initramfs-recovery-installer.itb
   image using sysupgrade.
4. Wait for installation: the green status LED will blink rapidly,
   indicating that the all-in-UBI installer is running.
5. Once the installation finishes,
   the status LED will turn solid amber for 5 seconds.
6. After the device reboots, perform a final sysupgrade using the
   openwrt*-ubi-squashfs-sysupgrade.itb image.

Return to stock MTD
-------------------
1. Flash openwrt*comfast_cf-wr632ax-initramfs-kernel.bin
   via sysupgrade
2. Copy files to /tmp on the device via SCP:
   BL2.bin
   u-boot-env.bin
   Factory.bin
   FIP.bin
   openwrt*-squashfs-sysupgrade.bin
3. Restore stock MTD partitions:
   apk add kmod-mtd-rw
   insmod mtd-rw i_want_a_brick=1
   mtd write /tmp/BL2.bin BL2
   mtd write /tmp/u-boot-env.bin u-boot-env
   mtd write /tmp/Factory.bin Factory
   mtd write /tmp/FIP.bin FIP
4. Install the system:
   sysupgrade /tmp/*sysupgrade.bin

BL2 and FIP Recovery
--------------------
Use mtk_uartboot to recover corrupted BL2 or FIP via UART:
https://github.com/981213/mtk_uartboot

Stock layout
----------------------------------------
| dev:    size   erasesize  name       |
| mtd0: 00100000 00020000 "BL2"        |
| mtd1: 00080000 00020000 "u-boot-env" |
| mtd2: 00200000 00020000 "Factory"    |
| mtd3: 00200000 00020000 "FIP"        |
| mtd4: 07000000 00020000 "ubi"        |
----------------------------------------

OpenWrt U-Boot UBI layout
----------------------------------
| dev:    size   erasesize  name |
| mtd0: 00100000 00020000 "bl2"  |
| mtd1: 07f00000 00020000 "ubi"  |
----------------------------------

Signed-off-by: Andrii Kuiukoff <andros.ua@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/23981
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>