avr: atomic: reimplemented the atomic operations

Reimplemented most atomic operations so they rely on the most basic
version of themselves. For example, atomic_inc is basically an
atomic_add with 1.

New functions implemented:

	* atomic_add
	* atomic_sub
	* atomic_dec

Functions which have been rewritten:

	* atomic_inc

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