build: allow -j on top level makefile

By using $(MAKE) variable we allow to pass down to the called makefile
the number of jobs we want to compile with. The flags are communicated
to the next level via MAKEFLAGS variable.

Reference:
https://www.gnu.org/software/make/manual/html_node/Options_002fRecursion.html#Options_002fRecursion

Makefile

@@ -15,13 +15,13 @@ clean: plane copter rover antennatracker
 .PHONY: plane copter rover antennatracker
 
 plane:
-	make -C ArduPlane $(TARGET)
+	$(MAKE) -C ArduPlane $(TARGET)
 
 copter:
-	make -C ArduCopter $(TARGET)
+	$(MAKE) -C ArduCopter $(TARGET)
 
 rover:
-	make -C APMrover2 $(TARGET)
+	$(MAKE) -C APMrover2 $(TARGET)
 
 antennatracker:
-	make -C AntennaTracker $(TARGET)
+	$(MAKE) -C AntennaTracker $(TARGET)