Fixing Docs Makefile to correctly clean directories

Change-Id: I6494e67a3af488d8a903bf2a085bdbf88addb785
(cherry picked from commit 9bfa6f37fc74a1a4ece20d3e7ffbc77cba8f5417)
diff --git a/docs/Makefile b/docs/Makefile
index 8f616f9..ff4b7d6 100644
--- a/docs/Makefile
+++ b/docs/Makefile
@@ -25,7 +25,7 @@
 test: linkcheck
 
 linkcheck: build
-	linkchecker -a _book/
+	linkchecker --check-extern -a _book/
 
 platform-install:
 	ln -s ../platform-install/docs platform-install
@@ -59,5 +59,5 @@
 	rm -rf $(GENERATED_DOCS)
 	rm -rf _book
 	rm -rf node_modules
-	rm -f platform-install test profiles/* xos xos-gui xos-tosca
+	rm -rf platform-install test profiles xos xos-gui xos-tosca