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.
- use assert() for checking the line parameter
- use 'bit.h' for bitbanding
- simplified code a bit
- unified style of defifining the board configuration
- removed unused configurations form pba-d-01-kw2x and frdm-k64f
