cpu/$(CPU)/Makefile.features and cpu/$(CPU)/Makefile.dep are
automatically included
Part of moving CPU/CPU_MODEL definition to Makefile.features to have it
available before Makefile.include.
The Arduino Nano board is the cheapest member of the Arduino family and used the
same MCU as the Arduino Uno. It differs in the form factor (the Nano is much
smaller), it uses an integrated FT232RL TTL adapter instead of an ATmega16u2 to
provide access to the serial console via USB, and it uses a different
bootloader (which occupies 2 KiB of the 32 KiB flash instead of 0.5 KiB occupied
on the Arduino Uno). This commit mostly copy pastes code from the Arduino Uno.
