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.

Previous to this patch hexlify(b'', b':') would lead to a bad crash due to
the computed length of the result being -1=0xffffffff.