rootfs-builder: add functions to run before and after the container

Define `before_starting_container` and `after_stopping_container`
functions, these functions run before and after the container that
builds the rootfs respectively.

Signed-off-by: Julio Montes <julio.montes@intel.com>

tools/osbuilder/scripts/lib.sh

@@ -421,3 +421,11 @@ detect_musl_version()
 
 	[ "$?" == "0" ] && [ "$MUSL_VERSION" != "null" ]
 }
+
+before_starting_container() {
+	return 0
+}
+
+after_stopping_container() {
+	return 0
+}