Needs accompanying changes in Arduino's boards.txt, enabling corresponding menu entries

Sorry pasted in wrong code.

Migrated the T3 code over to handle the message to set the logical keyboard LEDS

As per forum thread: https://forum.pjrc.com/threads/59372-Teensy-4-0-no-support-for-keyboard_leds

SCB_ID_CSSELR is not const

Extracted from other PR.

This change only adds the code to look at the RX FIFO when you call available, and read/peak if our software queue is empty, it then checks to see if hardware FIFO has data to extract.

T4 IntervalTimer - change limit magic number

Longer term we might consider changing timer over to using system clock, with default(max) divisior of 4 to go to PID instead of fixed 24mhz...

But in the mean time, try to allow IntervalTimer Settings that work at least as well as default settings for T3.2 whoes BUS speed is 48mhz, so the magic number of 36 was used which allowed a minimum value passed into begin/update of 36
48*.75-.5

With T4 code we just blindly left the magic number of 36...  So this one simply sort of halves it.  24*.75-.5 ... I rounded down to 17

fixes #418

fixes #293, fixes #414

T4 IntervalTimer Priority

If you call set priority it was only looking at priority of first two items, even though there could be 4.  The main code that started an interrupt checked all 4. So I sort of copied that code over, only change was to simply set the top_priority to the first item to start with instead of 255 and then look at the first item

Update imxrt.h

Add T4 ADC_ETC definitions for bits plus structure

Decided to go to the format that is closer to the the manual to
and use names like: chain_1_0, chain_3_2...

https://forum.pjrc.com/threads/58667-Temboo-library-error-Unknown-type-name-bool

T4 - Can now handle sketches with main() instead of setup()/loop()

Add ADC Structure and redefine using, plus XBAR defines.

Update imxrt.h for encoder structure

Add structure to make it easer to use ADC objects,

Plus updated defines to use the structure.
Left in my own define for structure location as a trigger to not define it in my own code when running older version of core...

Also added defines for XBAR, such that we can hopefully not have to use magic numbers to say, map PID1 to ADC_ETC1_...