Add APFPV mode switching

[henkwiedig]

This PR will add APFPV mode switching for SBC 2 beta 2 and greg's APFPV image.
Please remove/uncomment these lines in stream.sh:

# sudo systemctl is-active --quiet wifibroadcast || sudo systemctl restart wifibroadcast
# sudo systemctl is-active --quiet wifibroadcast@gs || sudo systemctl restart wifibroadcast@gs

apfpv-demo.mp4

[Lupinixx]

Build locally and tested it on my Runcam VRX. Works as expected, switched between APFPV and wfb-ng from the menu, did a flight with both.

[zhouruixi]

1. Should we store APFPV settings in the pp_rk configuration file instead of retrieving them from other software configurations via commands?

2. Would it be simpler and more flexible to use wpa_supplicant instead of networkmanager to manage connections?

[seriyps]

sorry, I created a conflict with my other PR. But I think a better way to resolve it would be to not start WFB thread at all in main() (force-set wfb_port to 0 or have a dedicated flag for it).

[seriyps]

UPD just realized that RXMODE can change at runtime...