include/package-dumpinfo.mk: introduce Repository values

Introduce a new dumpinfo field "Repository" which contains the name of the
shared repository where the package will be hosted in. The name is chosen
according to the following rules:

 - Name of the feed directory for any feed package, e.g. "Repository: luci" for
   "feeds/luci/applications/luci-app-firewall/Makefile"
 - The value "base" for any other, sharable package
 - Unset for any package flagged PKG_FLAGS:=nonshared

Signed-off-by: Jo-Philipp Wich <jo@mein.io>