econet: Add new target TP-Link Archer VR1200v (v2)

econet: Add new target TP-Link Archer VR1200v (v2)

The TP-Link Archer VR1200v (v2) is a low end DSL modem based on the
EcoNet EN751221 processor platform.

While it does have an unlocked bootloader, the factory upgrade feature
requires a cryptographic signature so flashing from the web UI is not
feasible.

The Archer VR1200v (v2) uses a dual-image layout. I have chosen to reuse
this to support dual-boot between OpenWRT and the factory firmware.

Flashing instructions (from bootloader):

Build and then locate the squashfs-sysupgrade.bin image file
Get the length of that file in hex: printf '%X\n' "$(stat -c%s the-file-squashfs-sysupgrade.bin)"
Connect to device with xmodem capability, e.g. picocom --send-cmd lsx -vv -b 115200 /dev/ttyUSB0
Switch device on and press a key within 3 seconds, you should get to a `bldr>` prompt
Type: xmdm 80020000 <file length hex>
Quickly start xmodem and send the file, in picocom that is ctrl+a ctrl+s <paste-the-file-name> enter If the transfer fails to start, wait 30 seconds to a minute for the bootloader prompt to return and then try the command again.
Once the transfer has completed successfully, type the following flash 80000 80020000 <file length hex>
Type `re` or simply restart the device to boot into OpenWRT

Signed-off-by: Caleb James DeLisle <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/19021
Signed-off-by: Hauke Mehrtens <[email protected]>