Add TMCSTEPPER_SW_SERIAL to force usage of SW/HW serial

[skruppy]

This PR adds a method to force the usage of HW/SW serial, while still maintaining backwards compatibility. This new method can be used together with small changes to Marlin to automatically select the best method.

Related issues:

• Only including SoftwareSerial when actually necessary? #48 (with the small Marlin patch, your comment gets a solution)
• Disable softeware serial based on external definition. #228 (same problem, further down the release pipeline)
• [BUG] TMC2208 RXTX_pin not declared when SW_CAPABLE_PLATFORM not defined MarlinFirmware/Marlin#16299 (this was a bug never the less, but it got more visible with my changes, so I had to fix it too)

Aspects of the PR:

• Don't break code / be backwards compatible
• Changes not tailored specifically for Marlin (don't "detect" the presence of Marlin and behave appropriately)
• Prevent future name clashes: prefix / scope the new macro with the lib name TMCSTEPPER_
• Allow forcing "HW serial only" mode as well as "SW serial" mode (with a single macro)

If you are planing to release a 0.7.4 with this PR, then I will file the corresponding Marlin PR. If you don't plan to release any further v0 releases, in favor of v1, please add a similar mechanism 😃

[teemuatlut]

I can merge this in with 0.7.4 in mind but for the v1.0 branch I'd like to have something that doesn't rely so much on the preprocessor and works across IDEs and frameworks.

[phizev]

@skruppy thank you for this effort, I've been happily using this after failing to find a solution myself.

I've added a suggestion to the commit with the changes needed on Marlins side which accounts for a sanity check between software serial, and endstop interrupts.

[KevWal]

Thankyou everyone for the great work. I'd also like to use TMCStepper with Hardware Serial (so I don't need Software Serial) and have ENDSTOP_INTERRUPTS_FEATURE turned on in Marlin. Is it possible to progress a resolution to this? Thanks very much.

[phizev]

@KevWal if you patch this library with this PR, and apply the changes detailed in skruppy/Marlin@c53622b to Marlin, you can use hardware serial. I can't recall the exact steps involved to use a patched version of a library when compiling Marlin.

I wouldn't hold my breath on this being resolved on both sides. There are blockers raised in the Marlin issue which I do not understand, with questions which remain unanswered in terms of an acceptable path forward.

I've since moved to Klipper as I got tired of patching multiple projects to have basic functionality with seemingly no resolution in sight. I think the affected audience is just too small for this to be prioritised.