rpm/configure: Initialize repository metadata in SRPMS directory

We now generate repository metadata after building SRPMs - this
change silences a warning about missing metadata printed because
the metadata does not exist and can't be updated after the first
SRPM is built.

Signed-off-by: Euan Harris <euan.harris@citrix.com>
Merge pull request #436 from euanh/master

rpm/configure: Initialize repository metadata in SRPMS directory