mediatek: add alternative stock layout for Xiaomi Redmi Router AX6000

In this implementation, the flash partition layout is adjusted to avoid
modifying the uboot environment of mtdparts. This ensures that the 30M
ubi_kernel partition remains aligned with the stock ubi partition, and
the kernel volume is placed in it. This allows the stock uboot to boot
from it without changing the mtdparts, which is useful for reverting back
to the stock firmware using Xiaomi Firmware Tools. In actual testing,
modifying mtdparts has been found to break Xiaomi Firmware Tools.

1. use ARTIFACTS to generate initramfs-factory.ubi for easy installation.
2. The NAND flash layout is changed to allow for reverting back to the
   stock firmware.
3. Before performing sysupgrade, do some cleanup in platform_pre_upgrade
   to ensure a clean installation of OpenWRT.
4. Setup the uboot env to ensure that the system always boot, which can
   be helpful for users who may forget to do this before sysupgrade in
   the initramfs.

New flash instructions:
1. Gain ssh access. Please refer to:
   https://openwrt.org/toh/xiaomi/redmi_ax6000#installation)

2. Check which system current u-boot is loading from:
   COMMAND: `cat /proc/cmdline`
   sample OUTPUT: `console=ttyS0,115200n1 loglevel=8 firmware=1 uart_en=1`
   if firmware=1, current system is ubi1
   if firmware=0, current system is ubi0

3. Setup nvram and write the firmware:
   If the current system is ubi1, please set it up so that the next time
   it will boot from ubi, and write the firmware to ubi:
```
nvram set boot_wait=on
nvram set uart_en=1
nvram set flag_boot_rootfs=0
nvram set flag_last_success=0
nvram set flag_boot_success=1
nvram set flag_try_sys1_failed=0
nvram set flag_try_sys2_failed=0
nvram commit
ubiformat /dev/mtd8 -y -f /tmp/initramfs-factory.ubi
```
   If the current system is ubi, please set it up so that the next time
   it will boot from ubi1, and write the firmware to ubi1:
```
nvram set boot_wait=on
nvram set uart_en=1
nvram set flag_boot_rootfs=1
nvram set flag_last_success=1
nvram set flag_boot_success=1
nvram set flag_try_sys1_failed=0
nvram set flag_try_sys2_failed=0
nvram commit
ubiformat /dev/mtd9 -y -f /tmp/initramfs-factory.ubi
```

4. After rebooting, the system should now boot into the openwrt initramfs.
   Flash the squashfs-sysupgrade.bin via using ssh or luci.
```
sysupgrade -n /tmp/squashfs-sysupgrade.bin
```
Done.

For existing users of the Redmi AX6000 running OpenWrt, here are the steps to
switch to this new layout:

1. Flash initramfs-factory.ubi
```
mtd -r -e ubi write /tmp/initramfs-factory.ubi ubi
```

2. After rebooting, the system will boot into the new openwrt-initramfs.
Log in and perform a sysupgrade to complete the process.
```
sysupgrade -n /tmp/squashfs-sysupgrade.bin
```

Signed-off-by: Chen Minqiang <ptpt52@gmail.com>
Signed-off-by: Chuanhong Guo <gch981213@gmail.com>

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