Split atmega_common code into avr8_common folder. This moves common
avr8 code to be used for all avr8 variants: tiny, mega and xmega.
Signed-off-by: Gerson Fernando Budke <nandojve@gmail.com>
The module cpu_atmega_common_cxx seems to be non-existing and not used. It is
unclear whether this slipped in by accident or if this was actually useful at
some point in time. In any case, the module is not present (anymore) and cannot
be used, so let's clean up the Makefile.
cpu.c and startup.c were redundant in most platforms, except for
atmega256rfr2. The common code is now in cpu/atmega_common/cpu.c
and cpu/atmega_common/startup.c. cpu_conf.h is also removed as
it's now in cpu/atmega_common/include thus shared by all atmega
based platforms.
