Allow the user to call setTX with the one hardware pin, and hopefully allow them to turn on Open Drain mode or turn it back off.  I don't currently have any test cases that use this, so not sure how well it works.

Page 982.
Fixes https://forum.pjrc.com/threads/54711-Teensy-4-0-First-Beta-Test?p=196233&viewfull=1#post196233
- maybe this is needed for T3x, too? too late here to check this now.

The FlexIO registers are 32 bits, the masks were all setup for 16 bits so the high word of registers were not updated

warning: 'flexpwm' may be used uninitialized in this function [-Wmaybe-uninitialized]
warning: 'qtimer' may be used uninitialized in this function [-Wmaybe-uninitialized]

This fixes the PFDs for PLL2 - by default, it has totally wrong values. This sets the frequencies according to the diagram on Page 676.

The same fix is needed for PLL3 - I have code (same structure), but it does not work - Do have have any hints for me?
Is PLL3 even running? I not, can we start it? Or, if i is running - at which frequency, and  is there any non-std config?

for micros() Use: ARM Synchronization Primitives, include of arm_math.h

Get systick_isr data safely without disable interrupts, uses systick_safe_read as indicator variable

micros() unchanged - minor systick_isr() diversion - one line added to systick_isr.

This saves a cycle - and checks both values to avoid _isr change of either value based on code/execution

Alternate micros() extends millis() using ARM_DWT_CYCCNT
> Also makes unused_interrupt_vector() weak to allow user override like Teensy3

Sketches that includes String objects did not compile, so simply copy the wstring.cpp from Teensy3 directory.

Likewise sketches that used stream functions like:
Serial1.readBytes would not compile as the Stream  sources were not included.

So  likewise copied over from Teensy3 directory

"noinline, noclone" has the opposite effect of beeing faster now on T4
Maye it would be best to use an empty define ? Then we can remove the section in the linker-file, too.

#define FASTRUN

QA quick and dirty import of the AVR emulation stuff for SPI registers.

So far none of SPCR is implemented
in SPSR only getting status of SPIF is setup
SPDR  reads and writes to LPSPI4...

So, it allows for some simple SPI write functions and wait for the write to complete and read the result.

WIth this Adafruit_ILI9341 graphic test runs

Gives more free DTCM. 
This is the first iteration - after USB and other things are done we should look at this, again.

I created a structure IMXRT_LPSPI_t
like what was done for UARTS.

I hopefully got all of the unused fields in properly to get the registers on the correct addresses.
Note: I did not update the other #define LPSPIn_ values but left them, likewise, I create a new define for each of the addresses.

That way we can double check I have everything properly aligned

Add some support for serialEventX calls when yield is called.

Currently keep table of which SerialX objects are Used (.begin called) and each has their own function which does the
if (Serial1.available()) serialEvent1();

Calling SerialX.end(), removes the call from the list.  Also the default weak pointer serialEventX functions call the disable function, as to only be called once as no one is actually processing them...

At some point will integrate this with EventResponder

Put in first pass at SerialX.end(),
Which should disable the Serial object, and I believe set the IO pins to be GPIO pins.

Also defined the extern void SerialEvent1() like functions in header in preperation for callbacks...

Hopefully will be able to get this such that only those objects that are used will be brought in by the system...