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.

cmake/configs/nuttx_omnibus-f4sd_default.cmake

@@ -0,0 +1,150 @@
+
+px4_nuttx_configure(HWCLASS m4 CONFIG nsh ROMFS y ROMFSROOT px4fmu_common)
+
+set(config_uavcan_num_ifaces 1)
+
+set(config_module_list
+	#
+	# Board support modules
+	#
+	drivers/barometer/bmp280
+	#drivers/differential_pressure
+	#drivers/distance_sensor
+	drivers/magnetometer/hmc5883
+	drivers/telemetry/frsky_telemetry
+	drivers/imu/mpu6000
+
+	#drivers/batt_smbus
+	#drivers/blinkm
+	#drivers/camera_trigger
+	drivers/gps
+	#drivers/irlock
+	#drivers/mkblctrl
+	#drivers/oreoled
+	#drivers/pca9685
+	#drivers/pwm_input
+	drivers/px4flow
+	drivers/px4fmu
+	drivers/rgbled
+	drivers/stm32
+	#drivers/stm32/adc
+	#drivers/stm32/tone_alarm
+	#drivers/tap_esc
+	#drivers/vmount
+	modules/sensors
+
+	#
+	# System commands
+	#
+	systemcmds/bl_update
+	systemcmds/config
+	#systemcmds/dumpfile
+	#systemcmds/esc_calib
+	systemcmds/hardfault_log
+	systemcmds/led_control
+	systemcmds/mixer
+	#systemcmds/motor_ramp
+	#systemcmds/mtd
+	systemcmds/nshterm
+	systemcmds/param
+	systemcmds/perf
+	systemcmds/pwm
+	systemcmds/reboot
+	#systemcmds/sd_bench
+	systemcmds/top
+	systemcmds/topic_listener
+	systemcmds/tune_control
+	systemcmds/ver
+
+	##
+	## Testing
+	##
+	#drivers/distance_sensor/sf0x/sf0x_tests
+	#drivers/test_ppm
+	##lib/rc/rc_tests
+	#modules/commander/commander_tests
+	#lib/controllib/controllib_test
+	#modules/mavlink/mavlink_tests
+	#modules/mc_pos_control/mc_pos_control_tests
+	#modules/uORB/uORB_tests
+	#systemcmds/tests
+
+	#
+	# General system control
+	#
+	#modules/camera_feedback
+	modules/commander
+	modules/events
+	modules/gpio_led
+	modules/land_detector
+	modules/load_mon
+	modules/mavlink
+	modules/navigator
+	#modules/uavcan
+
+	#
+	# Estimation modules
+	#
+	modules/attitude_estimator_q
+	modules/ekf2
+	modules/landing_target_estimator
+	modules/local_position_estimator
+	#modules/position_estimator_inav
+	modules/wind_estimator
+
+	#
+	# Vehicle Control
+	#
+	modules/fw_att_control
+	modules/fw_pos_control_l1
+	modules/gnd_att_control
+	modules/gnd_pos_control
+	modules/mc_att_control
+	modules/mc_pos_control
+	modules/vtol_att_control
+
+	#
+	# Logging
+	#
+	modules/logger
+	modules/sdlog2
+
+	#
+	# Library modules
+	#
+	modules/dataman
+
+	##
+	## OBC challenge
+	##
+	#examples/bottle_drop
+
+	##
+	## Rover apps
+	##
+	#examples/rover_steering_control
+
+	##
+	## Segway
+	##
+	#examples/segway
+
+	##
+	## Demo apps
+	##
+
+	## Tutorial code from
+	## https://px4.io/dev/px4_simple_app
+	#examples/px4_simple_app
+
+	## Tutorial code from
+	## https://px4.io/dev/debug_values
+	#examples/px4_mavlink_debug
+
+	## Tutorial code from
+	## https://px4.io/dev/example_fixedwing_control
+	#examples/fixedwing_control
+
+	## Hardware test
+	#examples/hwtest
+)