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.py

@@ -187,7 +187,8 @@ class Homing:
         # Notify of upcoming homing operation
         self.printer.send_event("homing:home_rails_begin", self, rails)
         # Alter kinematics class to think printer is at forcepos
-        homing_axes = [axis for axis in range(3) if forcepos[axis] is not None]
+        force_axes = [axis for axis in range(3) if forcepos[axis] is not None]
+        homing_axes = "".join(["xyz"[i] for i in force_axes])
         startpos = self._fill_coord(forcepos)
         homepos = self._fill_coord(movepos)
         self.toolhead.set_position(startpos, homing_axes=homing_axes)
@@ -231,7 +232,7 @@ class Homing:
                                        + self.adjust_pos.get(s.get_name(), 0.))
                         for s in kin.get_steppers()}
             newpos = kin.calc_position(kin_spos)
-            for axis in homing_axes:
+            for axis in force_axes:
                 homepos[axis] = newpos[axis]
             self.toolhead.set_position(homepos)