mk: wibbles clean up output a little

clean up extra slash in /mk//Arduino.mk
etags can be .PHONY so we dont get errors about overriding existing recipie

mk/apm.mk

@@ -1,7 +1,8 @@
 # find the mk/ directory, which is where this makefile fragment
-# lives
-MK_DIR := $(dir $(lastword $(MAKEFILE_LIST)))
+# lives. (patsubst strips the trailing slash.)
+MK_DIR := $(patsubst %/,%,$(dir $(lastword $(MAKEFILE_LIST))))
 
+# APPDIR is defined for the PX4 build only
 ifeq ($(APPDIR),)
 
 ####################

mk/targets.mk

@@ -61,6 +61,7 @@ obc: apm2
 sitl-mount: EXTRAFLAGS += "-DMOUNT=ENABLED"
 sitl-mount: sitl
 
+.PHONY: etags
 etags:
 	cd .. && etags -f ArduCopter/TAGS --langmap=C++:.pde.cpp.h $$(git ls-files ArduCopter libraries)