Since firmware lives in a build directory which already specifies the
board name, no real reason to also have the firmware have the board
name.

BUILD is build-$(BOARD) by default.

This renames MICROPY_HW_HAS_WLAN to MICROPY_HW_ENABLE_CC3K (since it's a
driver, not a board feature) and wraps all CC3000 code in this #if.
It's disabled for all boards.

A negative ms/us is now treated as a delay of 0 ms/us.  This patch also
improves the calibration of udelay.

Make include dependencies neater, and adheres to the coding convention
that headers should not include headers.

This also adds a bunch of slashes separating pin that have multiple functions.

Which allows you to run "make -C stmhal flashboard" from the top-level directory.
Also ties in nicely with my configurable BUILD directory from #506

This documentation is now available at micropython.org/doc/

It's not enabled by default because it doesn't fully work.  It can
connect to an AP, get an IP address and do a host-lookup, but not yet do
send or recv on a socket.

Also give proper error message when trying to construct a non-existent
LED.

Addresses issue #530.

Decided to write own script to pull documentation from comments in C code.
Style for writing auto generated documentation is: start line with ///
and then use standard markdown to write the comment.  Keywords
recognised by the scraper begin with backslash.  See code for examples.

Running: python gendoc.py modpyb.c accel.c adc.c dac.c extint.c i2c.c
led.c pin.c rng.c servo.c spi.c uart.c usrsw.c, will generate a HTML
structure in gendoc-out/.

gendoc.py is crude but functional.  Needed something quick, and this was
it.

Instead of pyb.switch() as a function, it's more consistent (with
respect to all the other modules and peripherals) to have
pyb.Switch() create a switch object.  This then generalises to having
multiple switches.  Call the object to get its state.  Use sw.callback
to set the callback function for when the switch is pressed.

It should be a noun, since it supports getting and setting depending on
the number of arguments given.

Simple but functional timer control.  More sophistication will
eventually be added, or for now just use direct register access :)

Also added pyb.freq() function to get MCU clock frequencies.