Move hardware-specifics (pin assignments, power management) to
hwconfig_*.py.
The build system will build a firmware image
firmware-filesystem-$variant.uf2 for all variants for which a
hwconfig_$variant.py file exits. Inside the filesystem image, the
selected variants hwconfig_$variant.py file will always be named
hwconfig.py.
At runtime, main.py will attempt to import hwconfig which will load the
configuration for the correct variant.
Currently, the hwconfig_* modules are expected to define the pin mapping
and implement a board_init method.
Signed-off-by: Matthias Blankertz <matthias@blankertz.org>