- Set XTIMER_HZ to something that is actually possible to generate with one
of the available clock dividers from the core frequency
- Use xtimer_on_ztimer if xtimer is used and not ztimer_xtimer_compat is used
- This is needed because xtimer is simply not compatible with any of the
possible clock frequencies of this board
Only include its headers. Without this change board_init would be
defined twice. Once in `boards/waspmote-pro/board.c` and the other time
`boards/common/atmega/board.c`.
- 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`
