make cleandocs issue: rm -rf /output when obj not defined

In an attempt to get some readable documentation I cloned the git repository
to check how the PDF format is.

Entered Documentation and proceeded with:

	make pdfdocs

which failed, so to start fresh, my thought was that:

	make cleandocs

would be the next step before anything else. But, this resulted in:

	rm -rf /output

as $(obj) for some reason was empty.

Makefile for Documentation has:

	BUILDDIR      = $(obj)/output

	cleandocs:
		$(Q)rm -rf $(BUILDDIR)


This should (at least?!) be:

	BUILDDIR      = ./$(obj)/output
or:
	$(Q)rm -rf ./$(BUILDDIR)


Best regards,