FMUv2: enable DMA on USART6 for 2nd GPS and expand buffer sizes

this improves performance for log transfer, and for 2nd GPS
ms5611: report P and T in ms5611 info
PPM channel count detection is now using a more paranoid approach.
more precise range conversion for SBus input signals
Non-destructive handling of failsafe signals, distinction between frame loss and signal loss.
This kind of handling might be moved upstream into the application, once alarms are propagated by the ORB system.

This change is compatible with RX failsafe settings, but does not rely on it (uses SBus flags instead).
First stab at implementing better RSSI based connection status estimation, still needs some work and testing
Improved S.Bus scaling based on scope measurements
Further improved S.Bus scaling
otp: fixed build warnings