econet: Add new target SmartFiber XP8421-B

econet: Add new target SmartFiber XP8421-B

The SmartFiber XP8421-B is a fiber modem which is available for $20 online
and has 512MB of memory, 256MB of SPI NAND flash and 2 USB 2.0 ports in
addition to ethernet, wifi and XPON.

Because EcoNet is not currently producing evaluation boards, the XP8421-B
stands in as a convenient, low cost, off-the-shelf, representitive example
of the capabilities of the EN751221 econet processor. This is also the
example board that is included in the upstream Linux patchset.

The XP8421-B, and apparently many other devices of this platform, use a
dual-image layout. I have chosen to reuse this to support dual-boot between
OpenWRT and the factory firmware. Certain design decisions were made with
the goal of not overwriting data that is used by the factory OS.

This commit also introduces a utility for switching between OS_A and OS_B
which are used for OpenWRT and Factory OS respectively.

Flashing instructions (from bootloader):

Build and then locate the squashfs-tclinux.trx image file
Get the length of that file in hex: printf '%X\n' "$(stat -c%s the-file-squashfs-tclinux.trx)"
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
Enter bootloader username and password: telecomadmin nE7jA%5m
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 go or simply restart the device to boot into OpenWRT

Signed-off-by: Caleb James DeLisle <cjd@cjdns.fr>
Link: https://github.com/openwrt/openwrt/pull/19021
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>