stm: Add support for ctrl-C to interrupt running Python.

Using PendSV interrupt at lowest priority, code can now raise an
exception during an interrupt by calling pendsv_nlr_jump.  The exception
will be raised when all interrupts are finished.  This is used to trap
ctrl-C from the USB VCP to break out of running Python code.
Merge branch 'iabdalkader-master'

Conflicts:
	stm/usb.c
	stm/usb.h