mk: Added a "help" target to the make system.

Now if developers type "make help" they get a useful message.

mk/apm.mk

@@ -16,6 +16,12 @@ include $(MK_DIR)/configure.mk
 
 else
 
+# short-circuit build for the help target
+ifeq ($(MAKECMDGOALS),help)
+include $(MK_DIR)/help.mk
+
+else
+
 # common makefile components
 include $(MK_DIR)/targets.mk
 include $(MK_DIR)/sketch_sources.mk
@@ -53,3 +59,5 @@ endif
 endif
 
 endif
+
+endif

mk/help.mk

@@ -0,0 +1,35 @@
+help:
+	@echo ""
+	@echo " ArduPilot Building"
+	@echo " =================="
+	@echo ""
+	@echo " The following web page has detailed information on building the code"
+	@echo "     http://dev.ardupilot.com/wiki/building-the-code/"
+	@echo ""
+	@echo " Before building a target you need to be in the target vehicle type directory"
+	@echo " e.g. ArduPlane, ArduCopter, APMrover2, AntennaTracker"
+	@echo " and run \"make configure\""
+	@echo ""
+	@echo " Most targets support a \"-upload\" extension to upload the firmware"
+	@echo " to a connected board.  e.g. \"make px4-v2-upload\""
+	@echo ""
+	@echo " Note that the px4 builds are NOT parallel safe, NO -j flag"
+	@echo ""
+	@echo " Targets"
+	@echo " -------"
+	@echo ""
+	@echo "  configure - Set up build for vehicle type"
+	@echo ""
+	@echo "  all - Build all targets"
+	@echo ""
+	@echo "  apm1 - the APM1 board"
+	@echo "  apm2 - the APM2 board"
+	@echo "  px4-v1 - the PX4v1 board"
+	@echo "  px4-v2 - the Pixhawk"
+	@echo "  pxf - the Beagle Bone Black (BBB) + PXF cape combination"
+	@echo "  navio - the RaspberryPi + NavIO cape combination"
+	@echo "  linux - a generic Linux build"
+	@echo "  flymaple - the FlyMaple board"
+	@echo "  vrbrain - the VRBrain boards"
+	@echo "  stil - the SITL Software In The Loop simulation"
+	@echo "  bbbmini - the Beagle Bone Black mini"