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: I6f387f4d3db9d208040688e972ec284d9c0b84ff
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>