Back out another error introduced with commit 02c33f66c5a8be774034cd40e4125e9323c7b4d8. Causes an infinite loop in up_lowputc().

Remove conditional logic form non-SAM4CM parts (Gregory Nutt)

Back out an error introduced with commit 02c33f66c5a8be774034cd40e4125e9323c7b4d8.  Causes an infinite loop in up_lowputc().  From Max Neklyudov.

SAML21 DMA: Fix a logic error.  Since the write back descriptors overly the base descriptors, we need to do some special things in order to correctly free any allocated descriptors

SAMD21: Add build framework for DMA support.  Nothing there yet except for skeletal logic taken from SAM3/4.