SPI.begin() variants

[vshymanskyy]

On some boards, SPI.begin() is used to set SPI pins:

SPI.begin(5,19,27,18);

Currently, this is hardcoded in the library, but it may be best to leave it for the user sketch.
https://www.thethingsnetwork.org/forum/t/big-esp32-sx127x-topic/10247/131

[terrillmoore]

Good idea. We can add this to the pin map, I think.

[cyberman54]

@terrillmoore I could need this, too, for my paxcounter software which supports generic boards.

[cyberman54]

Suggested implemenation with pull request #157

[terrillmoore]

Unfortunately, I discovered that my suggestion of adding to the pinmap can't work on Adafruit BSPs. Sigh. Plus (after suggesting that) I learned that changing the pinmap can be a major breaking change. See my comments on #157.

[terrillmoore]

This is closed by #190.