package.bbclass: add PACKAGE_ADD_METADATA to sstate variables

The PACKAGE_ADD_METADATA can be used to define additional metadata for
packages. Changes to this variable should affect the packaging sstate hash
so packages are re-generated when this variable changes.

This variable is added to both PKGDATA_VARS and PACKAGEVARS. It is needed
in PACKAGEVARS to ensure changes invoke do_package to re-run. It is needed
in PKGDATA_VARS to ensure changes are written into the pkgdata so that the
hash equiv reports a change and does not skip over the package_write tasks.

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