package_deb.bbclass: add PACKAGE_ADD_METADATA_DEB to sstate variables

Refactor the addition of vardeps for this package class to use DEBEXTRAVARS
and pass the items through gen_packagevar so package name specific overrides
are accounted for. This matches the style used in package_ipk.bbclass.

Additionally add PACKAGE_ADD_METADATA_DEB to the DEBEXTRAVARS list so the
packaging is redone if the variable changes value.

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