https://forum.pjrc.com/threads/28325-Teensy-LC-and-delay

strlen is already in string.h

or, alternativly one could write size_t strlen(const char *s) {return __builtin_strlen(s);} to use the gcc-builtin function.
I think, both are faster than the bytewise search.

millis()

systick_millis_count is already volatile, doing it twice is not needed.

Update kinetis.h

- Added bit definitions for the PIT
- limited Teensy LC to only two PITs
- added PIT_LTMR64 for Teensy LC

Update README.md

CS initialization corrected

Add some missing NVIC definitions

Fix for IDE Menu option "NO USB" @ F_CPU <= 16MHz LC/T3.x

Prevent inlining of "FASTRUN"

Fix Flash and Bus Clock dividers for F_CPU <= 16MHz for TeensyLC

https://forum.pjrc.com/threads/27093-Fix-for-9-bit-serial-receive-bug

(MKL26Z64) update for F_CPU <= 16MHz

https://forum.pjrc.com/threads/27803-Serial-paresInt-not-working-as-expected-with-Teensy-3-1

https://forum.pjrc.com/threads/27752-teensy-3-memcpy-has-gotten-slower