Add a universal migration script suited for migrating devices from the
removed ar71xx target to ath79.
While at it, try to abstract as much functionality possibly required in
the future for other migrations and implement it in the newly created
gluon.migration Lua package.
Signed-off-by: David Bauer <mail@david-bauer.net>
Moving the scripts to a common directory not only vastly simplifies the
zzz-gluon-upgrade script, but also allows to define an ordering of such
scripts across packages.
