scripts/package: allow custom options to rpm

Add a RPMOPTS make variable to allow arbitrary options to be passed
to rpm during 'make rpm-pkg'. For example:

 make RPMOPTS="--define '_topdir /home/jk/rpm'" rpm-pkg

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>