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.

Made the double touch portion a conditional compile based on the
PROBE_DOUBLE_TOUCH flag.

==============================================

Bugfix

The current G38 only stopped a move if it involved the Z axis.

Moved all the G38 code to it's own section and put it where it would
always be executed no matter what axis was moving or if the endstop was
enabled.

Also added a comment to configuration_adv to alert the user the double
tap had to be turned on.

==============================================

Change G38 back to using Z_MIN_PROBE

There's no Z_MIN endstop if Z_DUAL_ENDSTOPS is enabled and you have them
set to the top of the gantry.

G38 started out as using the Z_MIN_PROBE pin.  I don't remember why we
changed it to the Z_MIN endstop.

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