teensy 3.1 (mk20dx256) port broken.

[mbriday]

The Cortex port is now based on CMSIS and the port of the Teensy 3.1 is broken (out of date). I tried to import some CMSIS functions (NVIC_SetPriority is used for all ports). See commit 8502845
** It compiles (blink) but it is not tested on the real target **

Now interrupts are also broken (example isr1onFTM). Need help for this part :)

[jlbirccyn]

The Teensy port has been based on the code of Paul Stoffregen from PJRC and he does not use CMSIS. So the update will be longer than expected.