package_rpm.bbclass: respect package overrides for the main package

Apply ${PN} to OVERRIDES when determining the base package spec variables.
Without this, there is a mismatch in behaviour where overrides for the base
package have no effect while overrides for subpackages do. The subpackages
should not be affected by this override as they apply their own package
overrides and will by default fetch the base variables and apply them in
the spec file.

This makes the behaviour for package overrides match the implementation
in package_deb.bbclass and package_ipk.bbclass.

Signed-off-by: Michael Ho <Michael.Ho@bmw.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>