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
This define conflicts with LITTLE_ENDIAN defined in
include/machine/endian.h which is part of gcc-arm-none-eabi.
Also, the define does not seem to be used by the ASF included files.
The former i2c_init_master was too complicated. It was trying to set IN/OUT
mode of the pins, but all that is needed is to set the proper MUX.
Also the configuration for the boards was incomplete (no MUX, no pad
setting).
It was tested on a SODAQ Autonomo, but not on a samr21-xpro
