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.

On T3.2 if you do something like:
Serial2.setTX(31);
Serial2.setRX(26);
Serial2.begin(.9600);
...
Serial2.end();

It will on the end call will always reconfigure pins 9 and 10 to be
digital pins MUX = 1

These two functions were checking (SIM_SCGC4 & SIM_SCGC4_UART2)
and should be testing (SIM_SCGC4 & SIM_SCGC4_UART1)

Updated Serial2.
It had valid TX for T3.2 of 10 and 31, but 31 on T3.5 is RX4

Likewise RX for T3.2 of 9 and 26, but on T3.5 26 is RX1

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