If `RIOTBASE` is unset, this will expand to nothing.
Maybe execute [`RIOTBASE ?= $(abspath $(lastword $(MAKEFILE_LIST)))`][1] before that?
If `RIOTBASE` is empty/unset, set it to the folder the `Makefile.include` resides in, i.e. `.../RIOT`.

  [1]: http://stackoverflow.com/questions/18136918/how-to-get-current-directory-of-your-makefile