spiffs: Track spiffs submodule in component

Will now automatically initialise as part of "make".

components/spiffs/component.mk

@@ -1,3 +1,5 @@
 COMPONENT_ADD_INCLUDEDIRS := include
 COMPONENT_PRIV_INCLUDEDIRS := spiffs/src
 COMPONENT_SRCDIRS := . spiffs/src
+
+COMPONENT_SUBMODULES := spiffs