scripts: rework edtlib warnings-turned-errors

Create a "global" gen_defines.py option and edtlib.EDT constructor
kwarg that turns edtlib-specific warnings into errors. This applies to
edtlib-specific warnings only. Warnings that are just dupes of dtc
warnings are not affected.

Use it from twister to increase DT testing coverage in upstream zephyr.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>