package_rpm.bbclass: add PACKAGE_ADD_METADATA_RPM to sstate variables

Refactor the addition of vardeps for this package class to use RPMEXTRAVARS
and pass the items through gen_packagevar so package name specific overrides
are accounted for. This matches the style used in package_ipk.bbclass. Since
do_package_write_rpm does not have any "extra vars" that affect the rpm
packaging yet, this commit has to add the variable.

This refactoring is needed to add PACKAGE_ADD_METADATA_RPM to RPMEXTRAVARS
so it affects the sstate hash of do_package_write_rpm.

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