fix serial6_set_tx

fix serial5_set_tx and serial6_set_tx

fix limit checks readString/readStringUntil

https://forum.pjrc.com/threads/41282-Strange-looking-limit-checks-in-Teensy3-Version-of-Stream-cpp

https://forum.pjrc.com/threads/40893-Incorrect-return-for-Serial-write()-on-Teensy-3-6?p=127732#post127732

I'm not sure if this fix has some side-effects - how will other functions react when it does not return zero anymore ?

https://forum.pjrc.com/threads/40893-Incorrect-return-for-Serial-write()-on-Teensy-3-6?p=127764&viewfull=1#post127764

AudioStream.h: Allow inclusion from assembler .S files

Set RTS Watermarks dependend of rx-buffersize

Print.cpp: Add checks for nan, infm and ovr to increase arduino-compatibility

serialX buffersizes

Additional PR for SERIALX_DISABLE follows

Fix fcvtf warning in newer versions of newlib

Fix compilation error with recent newlib/gcc versions because of random()

Fix compilation warnings related to kinetis_hsrun_enable() for older teensies

fixed toolspath

The spaces after TOOLSPATH will append to the variable value. Merged the on Linux comment with previous comment to fix.

EEPROM write on T_3.6 under HSRUN - Drop and Raise