Provide a dtsi file that sets up common capabilities for all
Feather-based Particle Mesh devices. Provide additional dtsi files for
some obvious peripheral options.
Remove the xtensa esp32 image: it didn't build, and there's no
indication of how the ESP32 firmware can be updated on the Argon board.
Use particle_argon as the nRF52840 side of the board.
Add Particle Boron support.
Note that dtsi files must be replicated in each board directory until
tooling supports DTS includes from a shared area.
Signed-off-by: Peter A. Bigot <pab@pabigot.com>
