build: fix RPM builds

The rpmbuild tool requires certain directories to exist and will fail
rather than attempting to create them.  The 'clean' phase of maven will
remove these directories.

This patch adds support to ensure the directories exist.

NB. This patch changes the location of rpmbuild-related directories
from path/to/target/ (e.g. packages/fhs/target/RPMS) to
path/to/target/rpmbuild (e.g. packages/fhs/target/rpmbuild/RPMS).
There is an obvious knock-on effect to the location of resulting RPM
files.

Target: master
Request: 2.7 (as needed)
Request: 2.6 (as needed)
Request: 2.2 (as needed)
Patch: http://rb.dcache.org/r/6427/
Acked-by: Gerd Behrmann