Add FPort support

FPort code is mostly imported from Betaflight. To make future RX
updates easier, a few more changes have been done to make the
RX/Telemetry code in BF and INAV a bit closer.

- Imported RSSI handling code from BF with some changes
- frsky_vfas_cell_voltage setting renamed to report_cell_voltage
- Added telemetry_halfduplex setting
- sbus_inversion renamed to serialrx_inverted, should apply to all
protocols eventually
- New MSP cmds MSP_TX_INFO/MSP_SET_TX_INFO now allow setting the RSSI
via MSP, thus RX_MSP doesn't need to use an extra channel anymore.

src/main/build/debug.h

@@ -57,6 +57,8 @@ typedef enum {
     DEBUG_PITOT,
     DEBUG_AGL,
     DEBUG_FLOW_RAW,
+    DEBUG_SBUS,
+    DEBUG_FPORT,
     DEBUG_ALWAYS,
     DEBUG_COUNT
 } debugType_e;