Leverages common flasher (avrdude) and removes unnecessary exports.
Moreover, a reuse of serial.inc.mk is perfomed from the same
atmega_common/Makefile.include

* Remove '-Oihex' and '-Obinary' from OFLAGS for all boards
  It is now provided by the Makefile.include rule.

* Use the existing variable when possible
* Remove duplicate definition
* Remove unused BINFILE variable

All boards define a default PORT_LINUX and PORT_DARWIN
arduino-atmega-common uses PORT_LINUX and PORT_DARWIN accordingly

Refactors boards/arduino-common to boards/arduino-atmega-common.
Extends boards/arduino-atmega-common to be generic to all atmega based
arduino boards, not just atmega328p.
Removes flags that should be located in cpu framework