add RF.Guru RP2040 board

[ure]

add board https://github.com/Guru-RF/MicroPico

[ure]

@dhalbert i added your xtal start issue prevention sugention ;) Many thx !

[dhalbert]

@dhalbert i added your xtal start issue prevention sugention ;) Many thx !

You're welcome! For background on this see:
raspberrypi/pico-sdk#457
and raspberrypi/pico-sdk#401 (which takes a while to reach the right conclusion).

[ure]

@dhalbert thx ... interesting reading material ;) We use the RP2040 chip in many of our projects (10+ and counting ;)) and never ran into this issue ... luck must be on our side, hard fault on serial disconnect that was the first thing that really broke things ... but that's been solved in 9 also ... something to test in the weekend ;)

[dhalbert]

We thought it had to do with our choice of capacitor and crystal but redoing the capacitor values did not seem to fix things. Not that many boards had the trouble, but it was a noticeable small fraction util we add the startup delay.

[dhalbert]

Looks good!