allow betaflight passthrough when using serialrx_provider = MAVLINK

When I try to flash using passthrough, I get :

[ERROR] Invalid serial RX configuration detected:
!!! Serial Receiver Protocol is not set to CRSF! Hint: set serialrx_provider = CRSF !!!

... but I'm using "serialrx_provider = MAVLINK" (using USE_SERIALRX_MAVLINK during the betaflight 2025.12 build). So it would be difficult to set "serialrx_provider = CRSF" : I'd have to build and flash a new betaflight, flash ELRS and then build and flash back to MAVLINK.

I suppose the problem is that there are CRSF commands to put the ELRS rx into bootmode. Perhaps there are similiar MAVLINK commands.