Initial support for Netduino

Similar to mp_obj_new_tuple().

This also fixes up the IRQ for the PYBOARD4 USERSW
although I was unable to test that functionality.

Only works on modules and class instances.

Structure is back to expected 16 bytes.

Native emitter has been broken since stack order has changed from
reverse to standard.  This fix gets it partially working.

Takes slice object and sequence length and computes subsequence indexes
for case of slice step=1.

Both return int so far (single-precision float doesn't have enough
bits to represent int32 precisely).

IS_NOT and NOT_IN are now compiled to IS + NOT and IN + NOT, with a new
special NOT bytecode.

Still todo: break/continue from within the finally block itself.

Hopefully this works for other people as well :)

Add mpconfigport.mk file to configure which modules to include into build

Proof of concept, controls "ffi" module as one which requires external
dependencies.

Add exception stack unwind support for RETURN_VALUE.

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

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.

TODO: Decide if we really need separate bytecode for creating functions
with default arguments - we would need same for closures, then there're
keywords arguments too. Having all combinations is a small exponential
explosion, likely we need just 2 cases - simplest (no defaults, no kw),
and full - defaults & kw.