It's not used anywhere else in the VM loop, and clashes with (is shadowed
by) the n_state variable that's redeclared towards the end of the
mp_execute_bytecode function.  Code size is unchanged.

The code conventions suggest using header guards, but do not define how
those should look like and instead point to existing files. However, not
all existing files follow the same scheme, sometimes omitting header guards
altogether, sometimes using non-standard names, making it easy to
accidentally pick a "wrong" example.

This commit ensures that all header files of the MicroPython project (that
were not simply copied from somewhere else) follow the same pattern, that
was already present in the majority of files, especially in the py folder.

The rules are as follows.

Naming convention:
* start with the words MICROPY_INCLUDED
* contain the full path to the file
* replace special characters with _

In addition, there are no empty lines before #ifndef, between #ifndef and
one empty line before #endif. #endif is followed by a comment containing
the name of the guard macro.

py/grammar.h cannot use header guards by design, since it has to be
included multiple times in a single C file. Several other files also do not
need header guards as they are only used internally and guaranteed to be
included only once:
* MICROPY_MPHALPORT_H
* mpconfigboard.h
* mpconfigport.h
* mpthreadport.h
* pin_defs_*.h
* qstrdefs*.h

There are never any instances of these objects so there is no need to have
a print function.

Prior to this patch Servo numbers 1, 2, 3, 4 mapped to pins X3, X4, X1, X2
on PYBLITE which doesn't match the standard PYB mapping.  This patch fixes
the mapping.

The default frozen modules are no longer included (but users can still
specify their own via FROZEN_MPY_DIR), complex numbers are disabled and so
are the native, viper and asm_thumb emitters.  Users needing these features
can tune the build to disable other things.

gc_free() expects either NULL or a valid pointer into the heap, so the
checks for a valid pointer can be turned into assertions.

Reasons to disable:
- the code is relatively robust so doesn't need full checking in the
  main executable, and the coverage build is used for full testing
  with assertions still enabled;
- reduces code size noticeably, by 27k for x86-64 and 20k for x86;
- allows to more easily track changes in code size, since assertions
  can skew things.

These checks are already made, and errors reported, by the uPy runtime.

Non-blocking connect on the CC3100 has non-POSIX behaviour and needs to be
modified to match standard semantics.

This is required to easily giving links to a particular difference case.
Also, add RST anchors to allow cases to cross-reference each other.

This is the last "orphan" case.

CONF_FILE can be overriden, e.g. for minimal build, and we don't construct
such overriden conf file like we do for prj_merged.conf.

Got tired of running rm -rf manually. Make should clean, and should clean
fast. Also, fix always-running config-related commands (by having per-board
merged configs).

Adds nothing to the code size, since it uses existing empty slots in the
type structures.

Reduces code size.

This refactoring saves code space.

Code size change in bytes for this patch is:

   bare-arm:   -72
minimal x86:   -48
   unix x64:   -32
unix nanbox:  -120
     stmhal:   -68
     cc3200:   -64
    esp8266:   -56

This dev board only has a single USB connector, connected to the FS
peripheral.

The line in docs/conf.py with the release/version number was recently
changed and this patch makes the makeversionhdr.py script work again.