HelloRadioSky V12: expose the on-board UART header as an AUX1 serial port #7760

Is there an existing issue for this feature request?

• I have searched the existing issues

The UART4 header already came up in #7297: HelloRadio posted the PCB view
below, confirmed that UART4 is independent and usable for any function, and
said they would add it once V12 support was merged
(#7297 (comment)). V12
landed in #7114 without it, so this issue tracks the AUX1 part on its own.

Is your feature request related to a problem?

The V12 main board has a 3-pin GND/TXD/RXD header wired to UART4 (PH13 = TXD,
PH14 = RXD), but the target is built with AUX_SERIAL forced off:

set(AUX_SERIAL OFF CACHE BOOL "AUX serial port" FORCE)   # radio/src/targets/v12/CMakeLists.txt

So SYS → HARDWARE → Serial ports only offers USB-VCP, and the header cannot be
used for anything: no telemetry mirror to a Bluetooth module, no Lua serial,
no debug output, no GPS or S.Port. Because the CMake line uses FORCE, it
cannot even be switched on with -DAUX_SERIAL=ON in a local build. Companion
already assumes the V12 has an AUX port (HasAuxSerialMode is true for it
through IS_FAMILY_T16).

One thing to be upfront about: the header is inside the case. There is no
cut-out, so you have to take the back cover off to reach it. In practice a
Bluetooth or GPS module gets fitted once and stays inside; nothing needs to
be accessible from outside afterwards. The header itself carries only GND,
TXD and RXD; 3.3 V for a module is available from the top-left pin of the
unpopulated header to its right (the IMU footprint, which production units
do not populate), so a module needs no wiring beyond those four pins.

Describe the solution you'd like

Enable AUX_SERIAL on the V12 target and define the port in hal.h:

UART4, PH13/PH14 and DMA1 streams 2 and 6 are all unused on this target
today, so nothing else has to move. No Companion or YAML change is needed.

I have this working and will open a PR.

Question for the maintainers: is an internal-only header acceptable as an
AUX port in main, or do you want AUX ports to be reachable from outside the
radio? If always-on is not wanted for an internal header, I can make it a
plain option(AUX_SERIAL ... OFF) instead, so it can at least be enabled in
a local build; today the FORCE blocks even that.

Describe alternatives you've considered

1. USB-VCP only: needs a host computer on the cable, no use for a Bluetooth
   telemetry bridge or an on-board GPS.
2. Leaving the header unused: wastes a UART that is already on the board.
3. Keeping AUX_SERIAL off by default but removing the FORCE, so people who
   open the radio can enable it in their own build. Workable, but it keeps the
   port out of the official and cloud builds.

Additional context

The header on the main board (PCB view posted by HelloRadio in #7297):

Tested on a HelloRadioSky V12 (STM32H750) with a DX-BT24 Bluetooth module on
the header:

• Telemetry mirror at 115200 to an Android telemetry app (Ghost module).
• Lua serial in both directions: serialWrite() / serialRead() exchanging
  AT commands and replies with the module.

The same change applies to the 2.12 branch, where the V12 target also exists
(#7113).