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.

Additional PR for SERIALX_DISABLE follows

With the new beta boards, for those users who don't need an SDCard for
their project, it would be great to allow them to use the 6 IO pins
associated with the SDCARD for other purposes.  This would probably
require a cheap adapter, but these are nice signals to be able to be
used.  They include:

SPI1: SCK, MISO, MOSI, CS0-2
Serail2: TX/RX
Serial4: TX/RX
I2C1: SCL/SDA

This change adds support for GPIO, SPI, Serial2 and Serial4

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.

Was using UART2, should be UART3

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