package_rpm: correctly handle LICENSE_${PN}

package_rpm: correctly handle LICENSE_${PN}

RPM spec files don't distinguish between spec-level license and the primary
package license, so always output the License for every package (defaulting to
the recipe-wide license) and use LICENSE_${PN} if set for the primary package.

As the primary use of package_rpm is to generate built packages, this fixes the
license fields in the generated packages.

[ YOCTO #12057 ]

(From OE-Core rev: 8c333e592fa7a3a887c792ac14e5cde2141b5d35)

(From OE-Core rev: 42e0de37d18f072dc5dcf5dc45cb441e4c2110d8)

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>