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

Fix a few errors/warnings in THTTPD when indexing is enabled.  Still depends on non-existent lstat and ctime

STL32 F15x: stm32_stdclockconfig() was calling stm32_pw_setvos() which accessed PWR_CR via an inactive APB. From Juha Niskanen.

Add support for MK20DN--VLH5 and MK20DX---VLH5.  Needed for backward compatible support for Teensy-3.0