target_skeleton: mark empty directories with .empty files

Mark empty directories in target skeletons with .empty files
and remove them while copying to target (like it's already done for
svn and cvs files) to better support version control system which don't
handle empty directories (E.G. git-svn).

Makefile

@@ -330,6 +330,7 @@ $(PROJECT_BUILD_DIR)/.root:
 	fi
 	-find $(TARGET_DIR) -type d -name CVS | xargs rm -rf
 	-find $(TARGET_DIR) -type d -name .svn | xargs rm -rf
+	-find $(TARGET_DIR) -type d -name .empty | xargs rm -rf	
 	touch $@
 
 $(TARGET_DIR): $(PROJECT_BUILD_DIR)/.root