py/mphal.h contains declarations for generic mp_hal_XXX functions, such
as stdio and delay/ticks, which ports should provide definitions for.  A
port will also provide mphalport.h with further HAL declarations.

This prevents the loss of RTC time when exiting from standby mode, since
the RTC is paused while it is being re-inited and this loses about 120ms.

Thanks to @chuckbook for the patch.

This file is actually port-generic and should be moved out of stmhal/ .
Other ports already use it, and thus it should use mp_hal_ticks_ms()
right away.

These MPHAL functions are intended to replace previously used HAL_Delay(),
HAL_GetTick() to provide better naming and MPHAL separation (they are
fully equivalent otherwise).

Also, refactor extmod/modlwip to use them.

Addresses issue #1529.

mpconfigport.h is a private, partial header not providing correct settings
unless included by py/mpconfig.h.

Scenario: module1 depends on some common file from lib/, so specifies it
in its SRC_MOD, and the same situation with module2, then common file
from lib/ eventually ends up listed twice in $(OBJ), which leads to link
errors.

Make is equipped to deal with such situation easily, quoting the manual:
"The value of $^ omits duplicate prerequisites, while $+ retains them and
preserves their order." So, just use $^ consistently in all link targets.

Looks like we can use the same Pin class for legacy pyb module and new
machine module.

Instead of return 0, which means EOF. There's no good way to detect EOF on
continuously active bus like UART, and treat timeout as just temporary
unvailability of data. .read() method of UART object will return None in
this case (instead of 0, which again measn EOF). This is fully compliant
with unix port.

pyb.repl_uart still exists but points to os.dupterm.

pyb module still has pyb.delay and pyb.udelay, but these now point to
time.sleep_ms and time.sleep_us respectively.

Generally, ports should inherit INC from py.mk, append to it, not
overwrite it. TODO: Likely should do the same for other vars too.

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.

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.

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.

Thanks to Peter Hinch.  Addresses issue #1488.

Thanks to Peter Hinch.  See issue #1490.

With this patch parse nodes are allocated sequentially in chunks.  This
reduces fragmentation of the heap and prevents waste at the end of
individually allocated parse nodes.

Saves roughly 20% of RAM during parse stage.

Configurable with MICROPY_PY_BUILTINS_SLICE_ATTRS.  Disabled by default.

There is an issue sending 1 byte on the SPI bus using DMA, but it only
occurs when the transmit is done for the first time after initialising
the SPI and DMA peripherals.  All other cases (sending 2 or more bytes,
doing send_recv, doing recv first) work okay.  We sidestep this issue by
using polling (not DMA) for all 1 byte transfers.  This is fine because
a 1 byte transfer can't be interrupted and doesn't need the benefits of
DMA (and using polling for this case is more efficient).

Resolves #1456.

Use this to set the global optimisation value when executing the main
script (and all scripts it imports).

Fetch the current usb mode and return a string representation when
pyb.usb_mode() is called with no args. The possible string values are interned
as qstr's. None will be returned if an incorrect mode is set.