Makefile: Remove queueing script for repository database updates
Makefile: Create SRPM repository metadata after all SRPMS are created

It is not necessary to rebuild the SRPM repository metadata
after each SRPM is built.   Build it once, at the end.

Signed-off-by: Euan Harris <euan.harris@citrix.com>
Makefile: Add documentation

Signed-off-by: Euan Harris <euan.harris@citrix.com>
Makefile: Don't build the srpms repository by default

Currently the Debian and RPM builds share the rpms
and srpms targets, and the srpms_repo rule tries to
run createrepo which is not appropriate and will fail
on Debian.

The srpms_repo target is a convenience, not needed for
building binary packages.   It will return after we
separate the targets used for building .debs and .rpms.

Signed-off-by: Euan Harris <euan.harris@citrix.com>