Migrating the half duplex support I did in the Stagnant and out of date PR to enhance Uarts: https://github.com/PaulStoffregen/cores/pull/419

To try to support Half duplex mode in a similar way like we do for RS485 like support.

With T3.x made use of bitband address for the TX Direction setting, so
Except for the begin which calls format, no code changed, we simply stored the bitband address of the TXDIR flag into the TXDIR variable that was already used for the set direction flag.

For T4.x - it is a little more work as there is no bitband support on M7 processors.  For GPIO there is a Set and Clear register which we use, but for the appropriate UART register there is no such setup of registers.  So I have to special case we are in that mode and do it... Also since not atomic I cli/sei...

Serial Half Duplex - Fix T4Serial1 and T-LC Serial1-3

Updates: T4
T4 Serial 1 did not have proper settings for IOMUXC_LPUART6_TX_SELECT_INPUT

Tested T4.1 Serial1-8

Tested T3.5 Serial1-6

T3.6 - Added support for LPUART which is Serial6 which now works.

T-LC
Make half duplex work on T-LC on Serials1-3

Tested on on T3.5 1-6

Code is setup that if there are no eventResponders that have registered to do interrupt calls, don't have the systic code in place to look for it.

Also yield code.  Now figures out if the user has any code that installs their own serialEventX methods, to call them, but if they have left the default weak pointer versions, to not call them in yield.  Likewise will only look for eventRespnder events if the sketch has actually attached any events.  If none of these conditions are true, yield will just check one flag and return.

T3.x Serial events cleanup

The T-LC and the new beta boards T3.6 and soon T3.5 hardware uarts have
ability to turn on 2 stop bits.  So enabled on these boards.

This is a WIP,  But I have now been able to create Serial6 on T3.4 beta,
and so far have tested Send/Receive basic stuff at 115200 and have tried
at several CPU speeds in MHZ (192, 216, 180, 120, 96)

https://forum.pjrc.com/threads/29446-Teensy-Hardware-Flow-Control-RTS-CTS?p=80425#post80425

https://groups.google.com/a/arduino.cc/d/msg/developers/ls3hkviFYM4/XWT2LbfzNcgJ

http://forum.pjrc.com/threads/26064-Large-transfers-over-serial-on-Teensy-3-1
http://forum.arduino.cc/index.php/topic,165574.0.html

http://forum.pjrc.com/threads/25494-Problems-with-HardwareSerial-Pointer-and-write