Segger RTT and ARM Semihosting

[Avamander]

I would really like to add a bundled back-end for both Segger RTT and ARM Semihosting to this library.

The primary advantage of both approaches is the lack of need for anything else than the JTAG/SWD port access. Semihosting is something that's also accessible with open-source tools such as OpenOCD.

[2bndy5]

Should these extra interfaces only get built when build type is set to DEBUG? I imagine the build would fail if the end user doesn't have the proper dependency software/drivers installed. We could also employ a CLI arg to CMake to avoid requiring the user to satisfy these circumstantial dependencies.

[Avamander]

I imagine the build would fail if the end user doesn't have the proper dependency software/drivers installed.

The implementations I've seen are quite self-contained, but could be built only when explicitly enabled. Either way it's usually without external dependencies.

[2bndy5]

Clearly, I need to explorer semi-hosting more. I'll probably start with how PlatformIO implements debuggers... BTW, I have added a library.json (in the "2bndy5-refactor" branch) for compatibility with PlatfomIO, but I haven't actually tested this scenario since we'd be primarily relying on the Arduino framework.