toolhead: Pass set_position() homing_axes parameter as a string

Use strings such as "xyz" to specify which axes are to be considered
homing during a set_position() call.  This makes the parameter a
little less cryptic.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

klippy/extras/homing_override.py

@@ -46,11 +46,11 @@ class HomingOverride:
         # Calculate forced position (if configured)
         toolhead = self.printer.lookup_object('toolhead')
         pos = toolhead.get_position()
-        homing_axes = []
+        homing_axes = ""
         for axis, loc in enumerate(self.start_pos):
             if loc is not None:
                 pos[axis] = loc
-                homing_axes.append(axis)
+                homing_axes += "xyz"[axis]
         toolhead.set_position(pos, homing_axes=homing_axes)
         # Perform homing
         context = self.template.create_template_context()