The code to print a hex nibble got broke.   SERIAL_ECHO( (char) ) isn't
suitable for its use.

All the changes to get the code into compliance with Marlin coding
standards made UBL sick.   This pretty much gets UBL back up and
running.

Especialy against stepper ISR.
This is even more important when a minimum pulse width is set,
increasing the runtime of a babystep.

Sometimes after killing heaters are still on.

G29 P4 R O  should work now.

The retraction flag in UBL's G26 was conflicting with the FWRETRACT option.

All fixed!

Fix the logic change that happened when UBL got merged.

Thanks to @ospalh -- #6056