Support programming of Arduino devices in serial passthrough mode (#5129)

Support programming of Arduino devices in serial passthrough mode  (#5129)

* Support DTR in serial passthrough mode to enable programming of Arduino
based devices such as MinimOSD.

Use 'serialpassthrough 5 57600 rxtx 56' and then use Ardino to program MinimOSD
Use 'serialpassthrough 5 115200' and then use MWOSD configurator to setup

* Fix comment for CDC_SetCtrlLineStateCb routine

* Handle F7 CDC interface

* Use strToPin() to allow easy port/pin specification

* Fix use of CDC_SetCtrlLineStateCb for all processor types

* Only set baud when specified

* Fix unit tests for cli

* Only register callback if needed

* Fix white space

* Provide implementation of IOConfigGPIO in SITL

* Update serialpassthrough help text

* DTR handling through serial drivers

* Fix F3, F7 and SITL builds

* If serialpassthrough command specifies baud rate of 0, set baud rate over USB. MWOSD configurator can now access config and reflash MinimOSD without rebooting and changing baud rate.

* Fix F3 build

* Fix failing unit tests

* Use resources to declare DTR pin assignment

* Don't assert DTR during normal operation as MW_OSD doesn't like it

* MW_OSD must be built with MAX_SOFTRESET defined in order to support DTR resets

* Minimise changes after dropping DTR pin param from serialpassthrough cmd

* Remove DTR pin param from serialpassthrough cmd

* Treat ioDtrTag as boolean in conditional statements

* Tidy buffer check

* Check buffer size in CDC_Itf_Control

* Fix unit test

* Add documentation for DTR

* Add note on MAX_SOFTRESET to documentation

* Remove superfluous function definitions

* Fix tabs

* Fix tabs

* Removed superfluous entried from vtable

* Backout whitespace changes unrelated to this PR

* Pass true/false to IOWrite()

* Fix line coding packing

* Add LINE_CODING structure defintion

* Revise serial documentation

* Prevent tx buffer overflow in serialPassthrough()

* Revert change unrelated to PR

* Review feedback from ledvinap

* Fix unit test

* Use PINIO to drive DTR

* Fix unit test

* Remove change unrelated to PR

* Fix SITL build

* Use shifted bits for mask definition

* Fix serialpassthrough documentation

* Only compile PINIO functionality if USE_PINIO defined

* IOConfigGPIO not needed

* Move cbCtrlLine callback to cli.c

* serialPassthrough params changed

* Check packed structure size

* Fix unit test

* Tidy up baud rate handling