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.

platforms/nuttx/Images/omnibus-f4sd.prototype

@@ -0,0 +1,13 @@
+{
+    "board_id": 42,
+    "magic": "OMNIBUSF4SD",
+    "description": "Firmware for the OmnibusF4SD board",
+    "image": "",
+    "build_time": 0,
+    "summary": "PX4/OmnibusF4",
+    "version": "0.1",
+    "image_size": 0,
+    "image_maxsize": 1032192,
+    "git_identity": "",
+    "board_revision": 0
+}