This adds the board specification of the Adafruit Metro M4 Express [1].
The significance of this board is that it is compatible with both
classical SPI Arduino Shields using the ISP header for SPI
(such as `shield_w5100`) and more recent shields using D11/D12/D13 as
SPI (such as `shield_llcc68`).
[1]: https://learn.adafruit.com/adafruit-metro-m4-express-featuring-atsamd51/overview
