This adds support for all samd21 variants. Notice that the previous samd21
was moved to cpu/samr21
To use this cpu variant you must set at least the following in your boards
Makefile.include:
export CPU = samd21
export CPU_MODEL = samd21j18a # Not sure if this is used
CFLAGS += -D__SAMD21J18A__ # or whatever variant you have
The samd21 include files were copied from ASF 3.30
