Fixes #219

usb_serial.clear doesn't compile for F_CPU < 20 MHz

Allow definition of AUDIO_BLOCK_SAMPLES and AUDIO_SAMPLE_RATE_EXACT f…

Hi Paul, 
i know that defining the sample-rate from the commandline is - at the moment - not useful. 
But it is a first step to make ist possible :)
For AUDIO_BLOCK_SAMPLE, pls see this thread: https://forum.pjrc.com/threads/37131-Change-Audio-Block-Size

When clear function was added, it was not put in the null operations list when no USB enabled

https://forum.pjrc.com/threads/23681-Many-axis-joystick?p=41943&viewfull=1#post41943

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