- NuttX supports both versions
- POSIX shell only supports '='

Replaced with the more general serial config params.

Added an imu heater driver, formatted whitespace, standardized comments, and added doxy documentation.

 * tested on SPI (px4fmu-v4pro)
 * WIP I2C support

   Since the only device on the bus is the barro, int or ext does
   not apply. This will be fixed on the rev > RC15.
   This defines an alias that can be used to condition the buss logic
   connection logic px4_i2c_bus_external to return external.

Use 'if ! <cmd>' instead.

The Pixhack v3 comes with an external IST8310 in the GPS.

It's used further down again.

Updated and added full stops to comments and did some comment/whitespace formatting in the startup scripts.

It has an ICM-20602 IMU

Migrate LOGGER_BUF argument to rc.logging, group OUTPUT_MODE instances closer together by relocating UAVCAN logic block in rcS, and alphabetize hardware specific logic in rc.sensors.

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.

This change allows the LIS3MDL based GPS magnetometers to autostart on the mRo x2.1.

More information: https://leddartech.com/modules/leddarone/

disable mavlink on aero telemetry if tfmini enabled and start on mavlink companion device for others (needs SYS_COMPANION to be 0)

This allows to boot a Pixhack 3.0 which is the same as Pixhawk 2.0 / 2.1