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: I610d9179417bf30a0c813772564dc0ce9a5f8f7e
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>