Provide a common clock initialization driver rather than leaving
clock initialization to the boards code. A declarative description of
the board's clock configuration using a struct does still allow to
fine-tune settings. In addition, a board is still allowed to just
provide a custom `void clock_init(void)` if there really is the need
to do crazy things.
For this purpose, adapted AVR libc functions are used. When used, malloc and free functions require 304 additional bytes of code compared to the oneway_malloc module.
