This fixes builds after #4419. Thanks to @vbgl for the original commit;
I changed that as I'm not sure whether passing null values to buildInputs is clean.
CC maintainers: @coroa, @peti, @phreedom, @robberer, @jcumming.
This includes another source-only derivation for the web interface (and
patches the path into the twister binary), so there shouldn't be a need
to move it into ~/.twister/html separately.
Signed-off-by: aszlig <aszlig@redmoonstudios.org>
