- Include `tools/avrdude.mk` and `tools/serial.mk` at the common place instead
for each ATmega based board individually
- Introduce the makefile variable BOOTLOADER to de-duplicate flash configs:
- Two boards using the same bootloader now just use `BOOTLOADER ?= foo`
and share the individual config
- These settings are not applied when `PROGRAMMER` is set to still allow
users to manually specify how to program their board
- 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`
