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

SAML21: Since SERCOM5 usese a different output channel, it will also need a different GCLK generator

Add support uClibc++ excpetions.  This involves additional handling for relative relation types, additional support for unwinding, as well as additional changes.  The culmination of a big effort fromo Leo Aloe3132

Basic support for the Kinetis K20 architecture.  Taken from PX4.  This is the work of Jakob Odersky.

SAML21-Xlplained: Add options to enable XOSC32K and to use it as the DFLL source; NSH configure now uses DFLL with OSC16M source

Logic that samples the free running counter reads the pending interrupt status regsiter and can cause interrupts to be lost.  So, if when the status regsiter is read, the logic must also handle the timer overflow event.  Found and fixed by Max Neklyudov