The CPU variable in the boards Makefile.include file already contains the target
CPU, so there is no reason to provide it in each board again as avrdude flag.
This commit automatically sets the avrdude target from the CPU variable and
removes the unneeded flags.
- 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`
