fmu: add CRSF RC and Telemetry support

- Telemetry is only enabled on omnibus, since on Pixhawk it seems we cannot
  write to the RC UART due to how the board is wired
- For the Telemetry the UART needs to be opened RW

src/lib/rc/dsm.cpp

@@ -294,7 +294,7 @@ dsm_init(const char *device)
 {
 
 	if (dsm_fd < 0) {
-		dsm_fd = open(device, O_RDONLY | O_NONBLOCK);
+		dsm_fd = open(device, O_RDWR | O_NONBLOCK);
 	}
 
 	dsm_proto_init();