sdk: Add optional suffix to RPM release

This patch adds an optional "rpmsuffix" variable that will be added to
the RPM release number. The default value is empty. It is intended for
automatic build systems, so that the release number will be respected
and this suffix will be used to produce unique numbers.

Change-Id: I12f18a311e2f310a7d7092a622d9d32abacaf90c
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>