avr: init: abstracted some init calls.

Abstracted some config dependant init calls into defines. If the feature
in configured for building, it will process into the correct init call,
however if the feature is not configured for building it will process as
an empty define.

Signed-off-by: Michel Megens <dev@michelmegens.net>
avr: init: abstracted init calls in into a header.

Previously all config dependant init calls were defined in the AVR I/O
header, not the proper header for this kind of functionality.

All config dependant init calls are moved to a new header file;
<asm/init.h>.

Signed-off-by: Michel Megens <dev@michelmegens.net>