Tidy up keycode.h (qmk#3666)

* Tidy up keycode.h

* Fix spacing here

* Add back this comma

change messege at 'check-size' target

Add Atreus as an official board.

Rgb matrix arm (qmk#3648)

* Addition of I2C master driver for STM32, Generalization of ISSI3731 driver

- Addition of an i2c_master driver for STM32 to replicate expectations of AVR driver.
- Moved ISSI3731 driver one level up to make it accesible by both architectures.
- Renamed ISSI3731 functions to a more general name for preparation of other ISSI drivers.
- Added compiler directives where necessary to differenciate each architecture.

* converted tabs to spaces

Comment and organize build_keyboard.mk a bit

Split Common: Serial Backlight LED fix (qmk#3586)

Update ps2avrgb readme (qmk#3599)

* give ps2avrgb a specific readme

* add homebrew and crosspack avr step

* update readme of all ps2avrgb boards to reflect the new template

RGB Driver Documentation Update (qmk#3601)

- Minor formating in rgb matrix page
- Added a short discription for is31fl3731 in drivers page

Make sure 'shutdown_user' is called

Make sure 'shutdown_user' is called