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.

Make update of these flags be hopefully interrupt safe

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)

On Serial 3 this addition is for all 3.x.  Before it was ifdefed to only
do work on Teensy_LC as there is only one valid TX pin.  But we now pass
in open drain as an option so this code should allow you to not turn it
on/off on Serial3 on all of these boards

and on Serial4 and Serial5 on the new boards.

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

https://forum.pjrc.com/threads/27093-Fix-for-9-bit-serial-receive-bug

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