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.

It's really a UART because there is no external clock line (and hence no
synchronous ability, at least in the implementation of this module).
USART should be reserved for a module that has "S"ynchronous capabilities.

Also, UART is shorter and easier to type :)

The three classes I2C, SPI and USART now have a fairly uniform (Python)
API.  All are constructed, initialised and deinitialised in the same
way.  They can have most of their parameters set, using keyword arguments.
All have send and recv (although slightly different with I2C requiring an
address in master mode).  recv can do inplace receiving (ie store the
data in a previously-created bytearray).

It's just polling mode at the moment, but interrupt and DMA would be
nice to add.

The HAL handles for the I2C/SPI objects are rather large, so we don't
want to unnecessarily include them.