Build happens in 3 stages:

1. Zephyr config header and make vars are generated from prj.conf.
2. libmicropython is built using them.
3. Zephyr is built and final link happens.

Allows to get event time for a head item in the queue. The usecase
if waiting for the next event *OR* I/O completion. I/O completion may
happen before event triggers, and then wait should continue for the
remaining event time (or I/O completion may schedule another earlier
event altogether).

The new function has a strongly provisional status - it may be converted
to e.g. peek() function returning all of the event fields, not just time.

With the existing timeout of 100ms the transfer would end prematurely if
the baudrate was low and the number of bytes to send was high.  This patch
fixes the problem by making the timeout proportional to the number of bytes
that are being transferred.

This patch changes mp_uint_t to size_t for the len argument of the
following public facing C functions:

mp_obj_tuple_get
mp_obj_list_get
mp_obj_get_array

These functions take a pointer to the len argument (to be filled in by the
function) and callers of these functions should update their code so the
type of len is changed to size_t.  For ports that don't use nan-boxing
there should be no change in generate code because the size of the type
remains the same (word sized), and in a lot of cases there won't even be a
compiler warning if the type remains as mp_uint_t.

The reason for this change is to standardise on the use of size_t for
variables that count memory (or memory related) sizes/lengths.  It helps
builds that use nan-boxing.

With this patch all illegal assignments are reported as "can't assign to
expression".  Before the patch there were special cases for a literal on
the LHS, and for augmented assignments (eg +=), but it seems a waste of
bytes (and there are lots of bytes used in error messages) to spend on
distinguishing such errors which a user will rarely encounter.

By removing the 'E' code from the operator token encoding mini-language the
tokenising can be simplified.  The 'E' code was only used for the !=
operator which is now handled as a special case; the optimisations for the
general case more than make up for the addition of this single, special
case.  Furthermore, the . and ... operators can be handled in the same way
as != which reduces the code size a little further.

This simplification also removes a "goto".

Changes in code size for this patch are (measured in bytes):

bare-arm:       -48
minimal x86:    -64
unix x86-64:   -112
unix nanbox:    -64
stmhal:         -48
cc3200:         -48
esp8266:        -76

Saves 168 bytes on bare-arm.

It's needed by the DMA driver to do an efficient reinitialisation.  This
patch follows what is done in the F4 HAL.

On MCUs with a cache (eg F7) this must be done or else the SPI data that is
transferred is incorrect.

This keeps dependency on mdef, sysgen and other stuff which complicates
build integration.

The self variable may be closed-over in the function, and in that case the
call to super() should load the contents of the closure cell using
LOAD_DEREF (before this patch it would just load the cell directly).

Also const_table is now moved to the code_state->fun_bc structure.

Link references don't seem to work anymore.