Teensy4: Startup Hooks

fixes #394

Serial8 isn't a keyword now

https://forum.pjrc.com/threads/57360-Teensy4-IntervalTimer-Channels-Problem

Explicitly declare TMR channel registers in IMXRT_TMR_t

I suggest to explicitely delcare a (say)  IMXRT_TMR_CH_t struct which collects the registers for a TMR  channel instead of implicitely defining this struct in the declaration of IMXRT_TMR_t.

This would allow for an easier definition of user classes modelling a TMR channel.
Not a big deal, could also do that mayself the class code but it would be more convenient if this would be done in imxrt.h

Speed 24 MHz - not 24 Hz for Lowest voltage

CBT register for CANFD

Edited here on the web - copied to local machine and it built as tested : https://forum.pjrc.com/threads/54711-Teensy-4-0-First-Beta-Test?p=209959&viewfull=1#post209959

Proposed add locations for  startup_early_hook(); and startup_late_hook(); just before two existing waiting while()'s.
With debug 'PRINT_DEBUG_STUFF' disabled the three T4B2's here hit the EARLY code about 1.2 ms and LATE at about 45.3 ms after the systick is started.

Where the current printf_debug_init() call is might be critical at some point adjust 'something' as you did enabling the Serial4 port for debug - before clocks and other things are started perhaps startup_reset_hook()

Added the ability to parametrize Teensy 3 core USB init delays

T4 pwm sdcard pins

The pins were not in the table before...

rx_pins should depend on cnt_rx_pins not cnt_tx_pins, currentlty the same so does not impact

T4 Serial5 support setTx and setRx to pins 38, 39

Updated to get A12/A13 operational

Serial events main

This cut down some timings in yield.

Add quick test at start of function to return if there are no events pending.

Do this before checking if in ISR and the like.  Cut timing down a lot in yield.

Found some of our code has these defined, plus had in another test sketch so defined them hopefully with C bindings, but calls Serial1 object code

Put in probably temporary bandaids to allow code that uses:
Serial.readBytes(Buffer, cnt);

To function.  It simply loops calling the underlying code for Serial.read().

Comment out some of the main programs debug print statements plus add yield call to main loop.

yield can now call SerialEvent.

Put hack in that if the user has not overwritten the SerialEvent function, the weak linked one will turn off calling itself...