Names with two leading underscores are reserved in any context of the c
standard, and thus must not be used. This ATmega platform used it however for
defining internal stuff. This commit fixes this.
Moving atmega_stdio_init() to cpu_init() just before periph_init() guarantees
that stdio is available to allow DEBUG() in periph_init(). This also helps to
unify the boot up process of ATmega boards and de-duplicates the stdio init from
board_init().
Removes duplicated code for atmega platforms. They were all
basically the same, only with the exception of atmegarfr2,
for which there is an #if statement to use the code in the
same file.
