Update rate nosedives off a cliff about msp-osd

[bri3d]

This is because the dji_hdvt_uav process is also spamming the MSP processing logic with a gazillion MSP messages to send back over DUML.

I tried increasing the baud rate (the -f option in the OSD mux) thinking it was line saturation on the UART, but it's not, it's scheduling saturation on the MSP+serial tasks.

This can be proven/confirmed by stopping dji_hdvt_uav once the OSD is running - you will observe that the update rate recovers immediately. Of course, then you lose channel change, DJI telemetry, and arm/disarm monitoring at minimum, which is definitely undesirable.

What I need to do is write a "Fake FC" shim which sits in front of the map mux and responds to dji_hdvt_uav with fake/proxied data so that we're not letting DJI blast the MSP port.

[bri3d]

Fixed.