Initial omnibusf4sd target support

Flight tested: ekf2 w/ mag and compass by @nathantsoi: https://logs.px4.io/plot_app?log=79b81031-cf1e-41f0-890b-d6cd7d559766

NOTE: external I2C devices need a pullup. I have tested with a 3.3v 2.2k pullup.

Working:
 - mpu6000, bench tested and verified via nsh
 - fmu
 - all 6 ch output bench tested w/ pwm and oneshot via nsh
 - ppm input bench tested
 - dsm input bench tested
 - bmp280, bench tested and verified via nsh
 - hmc5883, bench tested and verified via nsh, but requires an external i2c pullup
 - gps on uart6
 - startuplog, nsh, mavlink on uart4, but params are not sent for some reason. RSSI pin is TX, MOTOR 5 is RX (normal mode, 57600 baud)
 - rgbled over i2c, bench tested and workingp
 - sbus via the shared sbus/ppm pin (which includes an inverter to the mcu SBUS in pin), remove the solder bridge or jumper to the ppm pin before use

Not yet implemented:
  - ADC
  - OSD: passthrough video is untested, use at your own risk until a basic driver is implemented.

ROMFS/px4fmu_common/init.d/rc.interface

@@ -120,7 +120,7 @@ then
 	fi
 fi
 
-if ver hwcmp MINDPX_V2 CRAZYFLIE AEROFC_V1 PX4FMU_V4 NXPHLITE_V3
+if ver hwcmp MINDPX_V2 CRAZYFLIE AEROFC_V1 PX4FMU_V4 NXPHLITE_V3 OMNIBUS_F4SD
 then
 	set MIXER_AUX none
 fi

ROMFS/px4fmu_common/init.d/rc.sensors

@@ -3,7 +3,7 @@
 # Standard startup script for sensor drivers.
 #
 
-if ver hwcmp AEROFC_V1
+if ver hwcmp AEROFC_V1 OMNIBUS_F4SD
 then
 	# Aero FC uses separate driver
 else
@@ -267,6 +267,16 @@ then
 	ll40ls start i2c
 fi
 
+if ver hwcmp OMNIBUS_F4SD
+then
+	mpu6000 -R 12 -s start
+
+	# Possible external compasses
+	hmc5883 -X start
+
+	bmp280 start
+fi
+
 if ver hwcmp PX4FMU_V4PRO
 then
 	# Internal SPI bus ICM-20608-G

ROMFS/px4fmu_common/init.d/rcS

@@ -13,6 +13,12 @@ set +e
 #
 # NOTE: COMMENT LINES ARE REMOVED BEFORE STORED IN ROMFS.
 #
+# UART mapping on OMNIBUSF4SD:
+#
+# USART1		/dev/ttyS0		SerialRX
+# USART4		/dev/ttyS1		TELEM1
+# USART6		/dev/ttyS2		GPS
+#
 # UART mapping on FMUv2/3/4:
 #
 # UART1			/dev/ttyS0		IO debug (except v4, there ttyS0 is the wifi)
@@ -229,6 +235,11 @@ then
 		set MAVLINK_COMPANION_DEVICE /dev/ttyS4
 	fi
 
+	if ver hwcmp OMNIBUS_F4SD
+	then
+		set MAVLINK_COMPANION_DEVICE /dev/ttyS1
+	fi
+
 	#
 	# Set USE_IO flag
 	#
@@ -281,8 +292,8 @@ then
 	#
 	if [ $AUTOCNF == yes ]
 	then
-		# Disable safety switch by default on Pixracer
-		if ver hwcmp PX4FMU_V4
+		# Disable safety switch by default on Pixracer and OmnibusF4SD
+		if ver hwcmp PX4FMU_V4 OMNIBUS_F4SD
 		then
 			param set CBRK_IO_SAFETY 22027
 		fi
@@ -412,7 +423,6 @@ then
 		sh /etc/init.d/rc.sensors
 		commander start
 	fi
-
 	send_event start
 	load_mon start
 
@@ -573,12 +583,12 @@ then
 			fi
 		fi
 
-		if ver hwcmp CRAZYFLIE
+		if ver hwcmp CRAZYFLIE OMNIBUS_F4SD
 		then
 			# Avoid using either of the two available serials
 			set MAVLINK_F none
 		fi
-	fi
+fi
 
 	if [ "x$MAVLINK_F" == xnone ]
 	then
@@ -707,7 +717,7 @@ then
 		fi
 	fi
 
-	if ver hwcmp PX4FMU_V2 PX4FMU_V4 PX4FMU_V4PRO MINDPX_V2 PX4FMU_V5
+	if ver hwcmp PX4FMU_V2 PX4FMU_V4 PX4FMU_V4PRO MINDPX_V2 PX4FMU_V5 OMNIBUS_F4SD
 	then
 		# Check for flow sensor - as it is a background task, launch it last
 		px4flow start &