- 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
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