cpu/$(CPU)/Makefile.features and cpu/$(CPU)/Makefile.dep are
automatically included
The multiple boards handling has been changed to use 'CPU_MODEL ?='.
Part of moving CPU/CPU_MODEL definition to Makefile.features to have it
available before Makefile.include.
Initially supports only Mulles with serial number > 220 (due to missing
MK60DN256ZVLL10 support in k60).
See also: https://github.com/RIOT-OS/RIOT/wiki/Board%3A-Mulle
Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>