[1.14] fmu-v6x: add crossfire UART driver

[MaEtUgR]

Solved Problem

When seeing https://youtu.be/LKpMfl6EHOQ?t=1107 I realized that the driver to support hooking up a Crossfire receiver to a UART port like Telem1/2, GPS2, ... which I happily use on a Holybro pixhawk 4 (fmu-v5) is not contained in the fmu-v6x build which made it not straight forward for Joshua in the video and also leads to the documentation suggesting to manually rebuild the firmware with the driver included: https://docs.px4.io/main/en/telemetry/crsf_telemetry.html#firmware-configuration-build

Solution

Add Kconfig target COMMON_RC for v6x which only contains the crossfire driver so far:

PX4-Autopilot/src/drivers/rc/Kconfig

Lines 2 to 5 in 071565a

	menuconfig COMMON_RC
	bool "Common RC"
	default n
	select DRIVERS_RC_CRSF_RC

Changelog Entry

Add Crossfire driver for direct FMU UART to fmu-v6x build

Alternatives

We could leave this but document and test they way of using Crossfire through the IO (without telemetry?) which should work out of the box but I never used.

Ideally, we eventually have a consistent way of using it documented across the supported boards.

Test coverage

I currently don't have a v6x board at hand to test but I could make that happen given enough time. On v5 it works flawlessly.

Context

It adds 5.66kb to the v6x release build and brings it from 99.3% to 99.59% flash usage which is not great for people adding customizations on top of the release because they will quickly need to remove. Also I don't add it to other boards so e.g. a pixracer (v4) could be next. Is the alternative better?