reduce by experiment to slowest before SWD on teensy burner stops working

reduce by experiement to slowest before SWD on teensy burner stops working https://github.com/adafruit/uf2-samd21/issues/16

Add support for Feather M4 Express

shumatech stack corruption fix; metro m4 updates; CPX neopixel glitch fix

Microsoft merge 2018 03 20

Update metro_m4 to rev F pins. Save rev B in metro_m4_revb.

Fix for stack corruption occurring when the bootloader executes an

applet.

SAMD51 support and other changes from adafruit version of uf2-samd21.

New Boards

Merges and np fix

1. use writenum() for serial number.
2. Add comment and VID checking for SWD programmer delay for Metro M0.
Also:
3. Make warnings be errors on compile so they don't fly by; fix remaining warnings.
4. `make all-boards` now stops when a board fails to build.
5. Fix .binfo location for Metro M4: M4 code is now >8K. Use <n>K constants instead of hex constants for readability.

USB power setting modifiable

String Descriptor Fix

Microsoft update 20180205

add pirkey and itsybitsy m0

change dotstar brightness to account for new 'dimmer' apa102's
update makefile to not use py3