scripts/package/Makefile: rpmbuild add support of RPMOPTS

After commit 21a59991ce0c ("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: 21a59991ce0c ("scripts/package/Makefile: rpmbuild is needed for rpm targets")
Cc: <stable@vger.kernel.org> # 4.3+
Notice: Have to remove the original signed-off-by line because gerrit
will notify the original author, which is not good. Keep the name and
the email address still.

Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Michal Marek <mmarek@suse.com>

upstream-status: backport

Change-Id: If7ce5d9d006ee9d49b2d7d200ebdc278395eb3f7
backport-by: Yunhong Jiang <yunhong.jiang@intel.com>
Signed-off-by: Yunhong Jiang <yunhong.jiang@linux.intel.com>