The board is deprecated, no need to ignore this board anymore here.
Having the `arduino` and `periph_pwm` features is now required for all
boards using 'arduino-atmega' (as it was except for that board).
If this should change in the future, it should be defined either in each
arduino board, in another board common, or per CPU_MODEL.
Features must be provided by the board if they're actually
available on board. Other features might be provided by the
CPU.
Some grouping is also removed as it is not necessary.
