[WIP] Further K64F modifications, attempting to get USB working

[fauxpark]

Since my last PR #297 I now have a Teensy 3.5. Unsurprisingly the first set of changes didn't work, so it seems there are more tweaks required to get USB functioning.

For the most part I've been referencing the PJRC Teensyduino code as well as the existing Kinetis implementations already in the repo (particularly the K66), as they're all very similar.
A simple QMK firmware is able to run and respond to key presses (qmk/qmk_firmware#14420) - I can reset it back to bootloader mode - however the K64F's USB peripheral refuses to cooperate. Looking at the data lines in a logic analyser, both of them appear to be pulled up for some reason, with no other activity.

@stapelberg @walkerstop since you've done some work getting the 3.6 running, would either of you have any insight into this? I must be missing something obvious...

[stapelberg]

@stapelberg @walkerstop since you've done some work getting the 3.6 running, would either of you have any insight into this? I must be missing something obvious...

Hm, no idea off the top of my head 🤔

Have you compared behavior with a known-working USB implementation on the Teensy 3.5? I don’t have a much better strategy than to compare behavior and any differences step by step…

[fauxpark]

My expectation was that Teensyduino would be a good reference - it doesn't seem to have a whole lot of chip-dependent code except for the obvious things like register definitions, which I've already checked. So I figured the existing USB LLD would also be more or less correct given that it looks like it's adapted from the Teensyduino stuff, and is used not just for the K66, but the K20xx chips as well.

[fpoussin]

Is this ready to be merged?
can you rebase it? tests were broken.

[fauxpark]

Thanks, rebased.

[fpoussin]

Thanks!