mem_info() is already pretty hacky, let it be more hacky.

I.e. with its own globals. So, just as for functions, we need to switch
globals when resuming a generator.

This follows pattern already used for objtuple, etc.: objfun.h's content
is not public - each and every piece of code should not have access to it.
It's not private either - with out architecture and implementation language
(C) it doesn't make sense to keep implementation of each object strictly
private and maintain cumbersome accessors. It's "local" - intended to be
used by a small set of "friend" (in C++ terms) objects.

Addresses issue #487.

Any generated headers go in $(BUILD)/genhdr/, and are #included as
'genhdr/xxx.h'.

Merge branch 'relocatable-build-dir' of github.com:lurch/micropython into lurch-relocatable-build-dir

nlr: Add implementation using setjmp/longjmp.

Add 'test' target to unix/Makefile

Allow the uPy used by run-tests to be overridden

Not all functions implemented.  Not enabled on pyboard.

Things get tricky when using the nlr code to catch exceptions.  Need to
ensure that the variables (stack layout) in the exception handler are
the same as in the bit protected by the exception handler.

Prior to this patch there were a few bugs.  1) The constant
mp_const_MemoryError_obj was being preloaded to a specific location on
the stack at the start of the function.  But this location on the stack
was being overwritten in the opcode loop (since it didn't think that
variable would ever be referenced again), and so when an exception
occurred, the variable holding the address of MemoryError was corrupt.
2) The FOR_ITER opcode detection in the exception handler used sp, which
may or may not contain the right value coming out of the main opcode
loop.

With this patch there is a clear separation of variables used in the
opcode loop and in the exception handler (should fix issue (2) above).
Furthermore, nlr_raise is no longer used in the opcode loop.  Instead,
it jumps directly into the exception handler.  This tells the C compiler
more about the possible code flow, and means that it should have the
same stack layout for the exception handler.  This should fix issue (1)
above.  Indeed, the generated (ARM) assembler has been checked explicitly,
and with 'goto exception_handler', the problem with &MemoryError is
fixed.

This may now fix problems with rge-sm, and probably many other subtle
bugs yet to show themselves.  Incidentally, rge-sm now passes on
pyboard (with a reduced range of integration)!

Main lesson: nlr is tricky.  Don't use nlr_push unless you know what you
are doing!  Luckily, it's not used in many places.  Using nlr_raise/jump
is fine.

Tests in basics (which should probably be renamed to core) should not
rely on float, or import any non-built-in files.  This way these tests
can be run when those features are not available.

All test in basics now pass on the pyboard using stmhal port, except for
string-repr which has some issues with character hex printing.

As discussed in #504

As discussed in #504

Needs a better solution.

Remove generated .h file if the generation process fails.

Fix to allow usbd_msc_storage.c to compile when MICROPY_HW_HAS_SDCARD is...

Fixing the USB problem on stmhal now gets these 2 tests working.

Taken straight from musl and newlib.  License seems compatible with MIT.

A 64-byte packet is now followed by a 0-byte packet if there is nothing
more to send.  This flushes the USB endpoint.

Internal flash used for the filesystem is now written (from the cache)
only after a 5s delay, or when a file is closed, or when the drive is
unmounted from the host.  This delay means that multiple writes can
accumulate in the cache, and leads to less writes to the flash, making
it last longer.

It's implemented by a high-priority interrupt that takes care of flash
erase and write, and flushing the cache.

This is still only an interim solution for the flash filesystem.  It
eventually needs to be replaced with something that uses less RAM for
the cache, something that can use more of the flash, and something that
does proper wear levelling.

Having an optimized asm implementation is good, but if we want portability,
that's it.

The autogenerated header files have been moved about, and an extra
include dir has been added, which means you can give a custom
BUILD=newbuilddir option to make, and everything "just works"

Also tidied up the way the different Makefiles build their include-
directory flags

In conjunction with #504 this allows you to do things like:
```shell
make -C unix clean && make -C unix test CC=gcc-4.7
```
all from the top-level micropython directory :-)

Something similar could probably be done for windows/Makefile too, but I don't have a cygwin setup to test with.

fix tests/bytecode/README to match contents of tests/bytecode/run-tests

with MICROPY_MP_PY envvar, in an analogous way to MICROPY_CPYTHON3 envvar.

(the reason for this will be made clearer by a later PR)