scripts/package/Makefile: rpmbuild add support of RPMOPTS

After commit 21a59991 ("scripts/package/Makefile: rpmbuild is needed
for rpm targets"), it is no longer possible to specify RPMOPTS.
For example, we can no longer able to control _topdir using the following
make command.
make RPMOPTS="--define '_topdir /home/xyz/workspace/'" binrpm-pkg

Fixes: 21a59991 ("scripts/package/Makefile: rpmbuild is needed for rpm targets")
Cc: <stable@vger.kernel.org> # 4.3+
Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Michal Marek <mmarek@suse.com>