Particle Xenon Support: Updates and fixes

[outlandnish]

Adds boards.txt info for flashing Xenon from Arduino IDE
Fixes pin mappings for digital pins
Can now control RGB LED and Primary LED with HardwarePWM and analogWrite()

[hathach]

Please remove these all xenon-specific functions, user should be able to do that in user sketch.

[outlandnish]

As mentioned in the changes, the primary and RGB leds now work with analogWrite and HardwarePWM

[outlandnish]

Added the Adafruit bootloader for the Particle Xenon to the bootloader folder to facilitate builds

[hathach]

Thank you for the huge progress so far, there is only a minor thing left to remove NRF52 macro, and check if SPI/WIRE pin mapping is correct. We can merge this after these changes.

[outlandnish]

@hathach @jaswope @DirtyEngineer thanks for pointing me in the right direction! Up until now I had changed just enough to get it to work for me but this helps a lot having it work properly.

[hathach]

Superb! Thank you very much for your effort so far. I am sure people will find this PR helpful

[jaswope]

Thanks for getting this off the ground. With Particle discontinuing support for the board, this is much needed. Please check out outlandnish#1 for a few more changes that I thought would be appropriate for this merge.

[outlandnish]

@jaswope yeah it's a bit sad that Particle is ending support (it's such a cheap and solid variant!) but it's really awesome they made as much of the documentation public as they did. That's what's really enabled any of this.

[hathach]

Please remove xenon from ci

[DirtyEngineer]

Just a question, Line 45 of variant.h
#define NUM_ANALOG_OUTPUTS (6)

Should this not be "0" like all of the other nRF52840 boards?

[outlandnish]

Just a question, Line 45 of variant.h
#define NUM_ANALOG_OUTPUTS (6)

Should this not be "0" like all of the other nRF52840 boards?

@DirtyEngineer Are PWM outputs not considered analog outputs? If not, I can set that to 0.

[DirtyEngineer]

@outlandnish It's my understanding that a true analog output would be from a DAC which to my knowledge the nRF52840 doesn't have. I think that the analog output field is legacy from the SAMD port which those chips do have a DAC. Anyone else have input on this?

[jaswope]

I've been cribbing off of the Feather nRF52840 Express variant for much of this, and it has 0 for the number of analog output pins as well. My bet is this is the same.

[jaswope]

In case it comes up: I defined the external flash device as a MX25L3233F (4MB flash), and I believe that to be correct. There's some differing information about which chip it uses when researching it, with some places saying it only has the 2MB MX25L1606 instead.

The reason I think the 4MB chip is correct is due to this blog post (which mentions the upgrade) and the datasheets included in the Xenon repo. I think the other chip is the initial design that was never shipped.

[outlandnish]

Sounds good, updated

[hathach]

Superb! Look great now, thank you very much everyone for working together on this PR.