stmhal: Update help text.

Remove reference to pyb.gc; add reference to pyb.millis.

There are lots of functions not listed when you run help(), but it would
be too much to list them all, so we list only some basic, useful ones.

Addresses issue #846.
stmhal: Set is_enabled=false when creating UART object; fix doc typo.
stmhal: Add basic CAN bus support.
tests: Add simple CAN test.
drivers: Add NRF24L01 driver (written in pure Python).

Comes with test script.  Copy both files to pyboard and run
"import nrf24l01test".
py: Convert [u]int to mp_[u]int_t where appropriate.

Addressing issue #50.
py: Use UINT_FMT instead of %d.
py: Change [u]int to mp_[u]int_t in qstr.[ch], and some other places.

This should pretty much resolve issue #50.
py: Fix unix-cpy to compile with uint->mp_uint_t changes.
Implement missing ARM emitter functions for viper
Merge pull request #889 from Vogtinator/master

Implement missing ARM emitter functions for viper
stmhal: Adjust computation of SYSCLK to retain precision.
stmhal: Allow pyb.freq() function to change SYSCLK frequency.

Eg pyb.freq(120000000) sets the CPU to 120MHz.  The frequency can be set
at any point in the code, and can be changed as many times as you like.
Note that any active timers will need to be reconfigured after a freq
change.

Valid range is 24MHz to 168MHz (but not all freqs are supported).  The
code maintains a 48MHz clock for the USB at all times and it's possible
to change the frequency at a USB REPL and keep the REPL alive (well,
most of the time it stays, sometimes it resets the USB for some reason).
Note that USB does not work with pyb.freq of 24MHz.
Implement kwargs for builtin open() and _io.FileIO

This makes open() and _io.FileIO() more CPython compliant.
The mode kwarg is fully iplemented.
The encoding kwarg is allowed but not implemented; mainly to allow
the tests to specify encoding for CPython, see #874
Use mode/encoding kwargs in io and unicode tests

mode argument is used to assert it works
encoding argument is used to make sure CPython uses the correct encoding
as it does not automatically use utf-8