ar71xx: add support for Alfa AP121F (#1404)

Adds support for Alfa AP121F USB powered travel router (looks like an USB ethernet adapter).
There is no factory image, but only a sysupgrade file for this router which can be installed using the inbuilt UBoot web interface.
Configure your PC with IP address 192.168.1.2 and connect the router using its RJ45 port. Hold the reset button while powering the device until all LEDs have flashed 3 times simultaneously. You can now access the fallback web interface using a web browser at http://192.168.1.1 and flash the sysupgrade file.

docs/index.rst

@@ -112,6 +112,7 @@ ar71xx-generic
 * ALFA Network
 
   - AP121
+  - AP121F
   - AP121U
   - Hornet-UB
   - Tube2H

targets/ar71xx-generic

@@ -16,6 +16,9 @@ factory
 
 # ALFA NETWORK
 
+device alfa-network-ap121f ap121f ap121f
+factory
+
 device alfa-network-hornet-ub hornet-ub HORNETUB
 alias alfa-network-ap121
 alias alfa-network-ap121u