...were missing :)

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.

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

fixes 136

fixes #138

https://forum.pjrc.com/threads/17469-millis%28%29-on-teensy-3?p=104924&viewfull=1#post104924

fixes #131

Signed-off-by: Jorg Neves Bliesener <jbliesener@bliesener.com>

On Teensy 3.x - we have Serial1.clear, Serial2.clear...

But not Serial.clear.

Serial.clear() exists for Teensy2, so thought I would try to add it for
Teensy 3s...