As defined by the new API, since 'auth' is actually a  tuple
composed by the security type and the key.

MICROPYPATH environment variable is now honored, package are installed to
first path specified in it.

This allows to build libffi from source together with micropython, and is
useful for cross-compilation. Support for this was already merged
previously, to use:

make libffi
make MICROPY_STANDALONE=1

(To both commands appropriate cross-compilition flags can be added).

Only available when MICROPY_CAN_OVERRIDE_BUILTINS is enabled.

Cortex-M0, M0+ and M1 only have ARMv6-M Thumb/Thumb2 instructions.  M3,
M4 and M7 have a superset of these, named ARMv7-M.  This patch adds a
config option to enable support of the superset of instructions.

Addresses issue #1182.

See issue #1500.

Bionic libc in Android 1.5 missed log2() and nan() functions.

Addresses #1510.

These are currently not intended to run with the rest of testsuite, as
they require dependencies and special environment setup anyway (drafted
in tests/jni/README).

Using generic iteration-via-subscription support (TODO: factor it out for
reuse).

This patch allows to configure the DAC resolution in the constructor and
in the init function, eg:

dac = DAC(1, bits=12).

The default resolution is 8 bits for backwards compatibility.  The bits
sets the maximum value accepted by write and write_timed methods, being
2**bits - 1.

When using write_timed with 12-bit resolution, the input buffer is
treated as an unsigned half-word array, typecode 'H'.

See PR #1130 for discussion.

As we dn't export constants for TCSANOW, etc., zero makes a good "don't
care" param, and now it will work also under Android Bionic and any other
libc.

The whole current port gets slurped into a static lib named
"libmicropython.a". Maybe that's not ideal, but at least something
to start with.

It makes much more sense to do constant folding in the parser while the
parse tree is being built.  This eliminates the need to create parse
nodes that will just be folded away.  The code is slightly simpler and a
bit smaller as well.

Constant folding now has a configuration option,
MICROPY_COMP_CONST_FOLDING, which is enabled by default.

Using 'P' format specifier (matches struct module). This is another shortcut
for FFI, just as previously introduced "array of objects" ('O').

Thanks to @nyov for the initial patch.

Use CTRL-E to enter paste mode.  Prompt starts with "===" and accepts
all characters verbatim, echoing them back.  Only control characters are
CTRL-C which cancels the input and returns to normal REPL, and CTRL-D
which ends the input and executes it.  The input is executed as though
it were a file.  The input is not added to the prompt history.

The usual cause would be that a cross-compiler for a port is not in PATH.

Apparently some cards need more than 2 retries.  See issue #1482.

This fix adds PIDs 9801 and 9802 to the pybcdc.inf file.

When in CDC only mode, it presents itself as a Communcations
device rather than as a composite device. Presenting as a
composite device with only the CDC interface seems to confuse
windows.

To test and make sure that the correct pybcdc.inf was being used,
I used USBDeview from http://www.nirsoft.net/utils/usb_devices_view.html
to uninstall any old pyboard drivers (Use Control-F and search
for pyboard). I found running USBDeview as administrator worked best.

Installing the driver in CDC+MSC mode first is recommended (since the
pybcdc.inf file in on the internal flash drive). Then when you switch
modes everything seems to work properly.

I used https://github.com/dhylands/upy-examples/blob/master/boot_switch.py
to easily switch the pyboard between the various USB modes for testing.