DIRDEPS_BUILD: Fix incorrectly adding in RELDIR for DIRDEPS in bootstrapping.

This is not wrong, but was unexpected. Using <empty>:H results in '.' which
then using the rest of the conversion was added in RELDIR. This was also
causing an empty _DP_DIRDEPS to resolve to SRCTOP for DIRDEPS.

Sponsored by: EMC / Isilon Storage Division