AVR (defined as __AVR__) should not be forced for all boards

[ianfixes]

Issue Summary

via #144 (comment)

__AVR__ is normally defined by avr-gcc, so only for the AVR-based Arduino boards. To emulate other boards (e.g. SAMD / Arduino Zero), I think __AVR__ should not be defined, otherwise libraries etc. will not be genuinely emulated and if they test for __AVR__ they will erronously assume that all kind of AVR-specific stuff is available.

Solution: remove __AVR__ from the hard-coded argument list in cpp_library.rb and add it to the appropriate board definitions in the default.yml CI config

[ianfixes]

This seems to be completed