...prevent overheating when overclocking and exiting loop()

Update mk20dx128.c re FSEC use on 3.5/3.6

RE forum post : https://forum.pjrc.com/threads/65070-Teensy-3-6-and-FSEC-security-register?p=262560&viewfull=1#post262560

Same FSEC change works on T_3.5 as well as the T_3.6

Fix for https://github.com/PaulStoffregen/cores/issues/499

Update PXP defines for T4

Update usb.c

configure usb_mtp also from usb.c

Create usb_mtp.c

Create usb_mtp.h

modified and adapted from rawhid

mtp file derived from rawhid and modified slightly

Update HardwareSerial.cpp

If called second time - fix the counts to reflect just the size of the new buffer plus the static buffer.

Also as a precaution, resets the TX or RX head tail pointers back to 0 to make sure it is not indexing into memory that may not exist.

T3 x serial x add memory

T3.x,T4.x LC Serial half duplex support

Configurable Midi Port Names

T4 No USB Support

Update usb.c

To allow future MTP_SERIAL implementations and facilitate use of MTP_t4, that provides possibility of MTP_SERIAL

Fix dumb mistake on my part - thanksKurt

Update usb.c so MTP_repsonder can work at 600Mhz

An experiement to see how much work again it would take to add the T4 methods to allow user sketches to increase the size of Serial ports RX and TX buffers, without having to edit the Source code which does it then for all sketches.

T3.x SerialN Add extra memory capabilities for Serial1-6

Also

Fixed serialEventX yield handling when you call begin multiple times

There is an issue that if you are not using the serialEvent type code and you call SerialX.begin multiple times, it will add that item to the process list multiple times, which can overrun the memory and cause crash or the like.

Fix was to check the list first and see if we are already in the list.

Update DMAChannel.h

Allow in both KINETISK and KINETISL DMA interrupt with priority
previous PR was only on one of them

T4.x Create Struct for IMXRT_GPIO

Wondered  if this had already been done earlier.

Was not so am trying it now.

One side effect was had to change interrupt.c as there were
#defines for things like DR, GDIR which then caused compile issues with the structure.  So renamed the defines.

Built about 3 programs with it so far.  Which appear to still be working.