makefiles: Select suitable terminal when stdio_rtt is used

[chrysn]

Contribution description

When stdio_rtt is used, the default value of RIOT_TERMINAL (pyterm) with some PORT makes little sense -- the board won't be using that.

Instead, when stdio_rtt is detected, the selected PROGRAMMER is introspected, and the used to pick the corresponding terminal transport.

Testing procedure

• Pick any board that is used with a debugger and usually runs serial stdio; I picked microbit-v2, and an example that has nice shell (eg. saul)
• Add lines to the Makefile:

  USEMODULE += stdio_rtt
  

• make -C examples/saul BOARD=microbit-v2 all flash term

(Note that this is identical to that of #18526 except that no RIOT_TERMINAL is set).

I've tried this with PROGRAMMER values of openocd and pyocd (for which RTT is not implemented, falling back to the pyterm default but issuing a warning); I'd appreciate if someone who uses JLink could test it with that too.

Issues/PRs references

Follow-up for #18526, based on a comment @benpicco left there.

[maribu]

ACK. Code looks good to me. I cannot provide a test with jlink, but given that it works fine for OpenOCD and the code is analogous, I am very optimistic it would just work.