Add support for ADB_TARGET specification.

This makes it easier to specify which target device the app
should be installed onto/uninstalled from. Sample usage:

	make install ADB_TARGET="-s emulator-5554"

xbuild.make

@@ -10,7 +10,7 @@ clean:
 	$(XBUILD) $(PROJ) /t:Clean $(EXTRA)
 
 install:
-	$(XBUILD) $(PROJ) /t:Install $(EXTRA)
+	$(XBUILD) $(PROJ) /t:Install $(if $(ADB_TARGET),"/p:AdbTarget=$(ADB_TARGET)",) $(EXTRA) $(INSTALL_EXTRA)
 
 uninstall:
-	$(XBUILD) $(PROJ) /t:Uninstall $(EXTRA)
+	$(XBUILD) $(PROJ) /t:Uninstall $(if $(ADB_TARGET),"/p:AdbTarget=$(ADB_TARGET)",) $(EXTRA) $(UNINSTALL_EXTRA)