toolhead: Rename _full_flush() to flush_step_generation() and use globally

Update code that modifies the low-level kinematics handlers to first
call toolhead.flush_step_generation().

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

klippy/extras/bltouch.py

@@ -144,6 +144,8 @@ class BLTouchEndstopWrapper:
         self.send_cmd(None)
         self.sync_print_time()
         self.mcu_endstop.home_prepare()
+        toolhead = self.printer.lookup_object('toolhead')
+        toolhead.flush_step_generation()
         self.start_mcu_pos = [(s, s.get_mcu_position())
                               for s in self.mcu_endstop.get_steppers()]
     def home_finalize(self):