boards/common/atmega: Refactoring periph_conf.h

- Moved code for periph_conf of all ATmega based boards to boards/common/atmega
- Added possibility to override config from individual board:
  - Named file `periph_conf_atmega_common.h` and let this be included from
    `board/$BOARD/include/periph_conf.h` to allow modifications
  - Guarded individual periph configs by `#ifndef $PERIPH_NUMOF` ... `#endif`