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* Avoid "chunkiness" of UART FIFO availability

The UART FIFO will generate an IRQ to transfer data into the SerialUART
FIFOs either every 4 received bytes, or every 4 idle byte times.  This
causes the ::available count to report "0" until either of those two
cases happen, causing a potentially delay in data becoming available to
the app.

Change the code to pull data from the HW FIFO on a read/available/peek.
Use a non-blocking mutex and IRQ disabling to safely empty the FIFO from
user space.  The mutex added to the IRQ is non-blocking and will be
a single CAS the vast majority of the time, so it should not impact the
Serial performance.

Fixes #464 and others where `setPollingMode()` was needed as a workaround.

Make sure we have all mutexes locked before we disable the port and free
the queue to avoid evil cases.

Only init the mutexes once, on object creation.

In polled mode, don't bother acquiring/releasing the fifo mutex.

When begin() is called on an already running port, call end() to clean
up the old data/etc. before making a new queue/config.  This avoids a
memory leak and potential write-after-free case.

Originally from https://github.com/episource/platform-raspberrypi/blob/my-platform-pico/builder/frameworks/arduino-pico/arduino-pico.py#L104-L115

Define ARDUINO_PICO_MAJOR/_MINOR/_REVISION for app use and update the
Platform.IO and Arduino files for a new release version.

Fixes #309
Fixes #487

* Added missing SERIAL2 and LoRa module GIO pins.
* Added support for enabling UART CTS and RTS pins.

The PIO and state machine were hard wired, so this caused problems if they were not free. The approach used by the Servo library has been adopted so a free PIO and SM are searched.

The DMA_IRQ_0 was grabbed exclusively, but this conflicts with SPI DMA use. The interrupt is now shared, but has been allocated the highest possible priority.

Since the PDM PIO use is receive only, the PIO state machine RX FIFO's can be joined to reduce DMA interrupt load.

Reorder the boards menu to make it easier to find any specific
board.  Add a note to the script asking manufacturers to keep it
that way.

Fixes #489

Only the PIO0 IRQ handler was ever installed due to a hardcoding bug.
Attach the IRQ handler to the PIO being started instead.

GDB for non-Linux systems was built w/o expat which caused odd behavior
under Windows and other systems (i.e. breakpoints not working, etc.)
New toolchain manually builds cross-compiled libexpat and ensures it is
used, fixing the issue.

Windows OpenOCD binaries now come from manually built and tested copies
(using a real Windows system).

Fixes #478
Fixes #457
Fixes #456
and probably others...

Fixes #472

Instead of using interrupts, explicitly call the IRQ handler dueing Serial
read/peek/available calls.

Add to keywords.txt for syntax hilighting.

Add poll calls in the SerialUART::write-like calls (write,
flush, etc.)

Really remove division from IRQ routines/

Microsoft is deprecating WMIC, so fall back to a Powershell call in
case of failure to ruin WMIC.

Belt and suspenders, set PowerShell non-interactive mode and null STDIN.

Fixes #468

The FIFO limit was set to 1/2, or 16 bytes on POR and not set by the core,
so for low baud this could result in a LONG time without data moving from
hardware FIFO to the SW ring buffer and timeouts/etc.

Now use the API call which sets it to 1/8, or 4 bytes of data to speed up
the transfer 4x.

Also avoid using the divider in the IRQ routine because it is not clear
from the docs of the Pico SDK IRQ callback routine preserves divider
state or not.  If not, doing division in an IRQ could result in random
data corruption in the main app.

Add memory barriers to ensure the order of data into RAM is preserved
and that GCC doesn't reorder writes.

Change order of tests to favor /run/media over /media. Parse output of udiskctl mount to find mount path

Fixes #464 and other incompatibilities

Remove the timeout check from ::read and ::peek on the SerialUART/PIO
classes  (SerialUSB already ignores it).  See documentation
https://www.arduino.cc/reference/en/language/functions/communication/serial/settimeout/
and thanks to @jandrassy's explanation
https://github.com/earlephilhower/arduino-pico/issues/464#issuecomment-1031657044

Most other boards and the MBED RP2040 support analogReadResolution which
just shifts read data around as needed, with a default of only 10b of
resolution.  The Pico ADC technically supports 12b, but only has about
8b of real data after noise, so you're not really losing anything in the
general case.

Fixes #460

Fixes #458

Fixes #453, a crash caused by Newlib having ARM instructions in
the Thumb-only Cortex-M0+.

Fixes #447

In order to be consistent the other tools in the directory.

 Correctly use earlephilhower.varant if variant not given, ensure USB power macro is always there (#444)

Fixes a crash of the builder script, `board.get("build.variant", None)` will throw an exception if the `build.variant` was not found (and not return `None` as the fallback value), but an empty string works, so check against that.

USB power defines moved directly into the board files, but still ensure that the macro always exists (with a default fallback value) to not fail the build.

As part of the boards.txt generation, also make necessary files for
Platform.IO.

Fixes #399