econet: Add Nokia G-240G-E and EN751221 recovery image

econet: Add Nokia G-240G-E and EN751221 recovery image

The Nokia G-240G-E is an xPON device with an EN7526G, 256M of
memory and 128M of flash. It has 1 USB2 port as well as phone and
ethernet but no wifi. Flashing instructions are per the typical
process using xmodem in the bootloader. This and other things
are described here: https://openwrt.org/inbox/toh/bt/g-240g-e_1

In addition, a generic image is offered, this image can be loaded
into memory from within the bootloader and launched directly. It
is recommended on the wiki of G-240G-E and other EcoNet devices
to be used for backing up the flash before flashing OpenWRT.

Signed-off-by: Caleb James DeLisle <cjd@cjdns.fr>
Link: https://github.com/openwrt/openwrt/pull/20338
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>