Add EnGenius EAP1300EXT support

The EnGenius EAP1300 and EAP1300EXT use identical boards and firmware (as flashed) from the vendor.

As with the EAP1300, the EAP1300EXT requires a specific firmware version to flash OpenWRT. Unfortunately, the required firmware is truncated on the vendor's website.

A working file can be created as follows:

curl \
https://www.engeniustech.com/wp_firmware/eap1300-all-v3.5.3.5_c1.9.04.bin \
| perl -pe 's/\x09EAP1300_A/\x0cEAP1300EXT_A/' \
> eap1300ext-all-v3.5.3.5_c1.9.04.bin

The file should have sha256: 58a1197a426139a12b03fd432334e677124cbe3384349bd7337f2ee71f1dcfd4.

Please see commit 2b4ac79 for further details.

The vendor firmware must be decrypted before it can be flashed from OpenWRT. A tool able to do that is available from:

https://github.com/ryancdotorg/enfringement/blob/main/decrypt.py

See also #15406 and #15484