Unify cpu_init for all Kinetis CPUs to reduce code duplication.
Updated the MCG driver implementation to make the configuration easier.
Most clock settings are initialized by kinetis_mcg_init() called from
cpu_init. Board specific external clock source initialization
(FRDM-K64F, PhyNode) needs to be performed in board_init instead of
in cpu_init.
The use of symbolic links leads to trouble in certain environments
(e.g. vagrant under Win). This PR gets rid of symlinks and uses
variables in the Makefiles to map to the correct linkerscripts
instead.
