edtlib: use standard logging module

The edtlib strategy for emitting warnings is to print directly to
standard error. This in turn requires hacks to drop stored references
to stderr in various _warn_file attributes so the EDT objects can be
pickled.

In general, I think it's not really appropriate for library modules
like edtlib to be printing to stderr directly. The user should be able
to configure logging for general utility data munging modules like
this as they please, and not just deciding what file to print to.

Move this around so the standard logging module is used instead. We
can preserve backwards compatibility in gen_defines by customizing the
'edtlib' logging module behavior so it prints the exact same thing it
always has.

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