Companion computer interface: Add SYS_COMPANION parameter. Setting it to 921600 enables the companion interface.

ROMFS/px4fmu_common/init.d/rcS

@@ -434,7 +434,10 @@ then
 	#
 	if ver hwcmp PX4FMU_V2
 	then
-		mavlink start -d /dev/ttyS2 -b 230400 -m onboard
+		if param compare SYS_COMPANION 921600
+		then
+			mavlink start -d /dev/ttyS2 -b 921600 -m onboard
+		fi
 	fi
 
 	# UAVCAN

src/modules/systemlib/system_params.c

@@ -82,3 +82,17 @@ PARAM_DEFINE_INT32(SYS_USE_IO, 1);
 * @group System
 */
 PARAM_DEFINE_INT32(SYS_RESTART_TYPE, 2);
+
+/**
+* Companion computer interface
+*
+* Configures the baud rate of the companion computer interface.
+* Set to zero to disable, set to 921600 to enable.
+* CURRENTLY ONLY SUPPORTS 921600 BAUD! Use extras.txt for
+* other baud rates.
+*
+* @min 0
+* @max 921600
+* @group System
+*/
+PARAM_DEFINE_INT32(SYS_COMPANION, 0);