wait for device initialisations
workaround for a timing issue during first boot on ath79-generic
after sysupgrade from ar71xx-generic image

GitHub Issue: #2779

Not useful by itself except for testing; will be used for multi-domain
support.

[Matthias Schiffer: rename script, use for initial configuration]

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.

As the (removed) comment said, this was a one-time-hack that was obsoleted long ago.

As we don't have any older gluon versions so far, and hope to be able to do all
upgrades with invariant scripts, remove the code for now.

If we ever see the need to add such upgrade scripts, we can just re-add this.

This is the most basic core of the Gluon framework. Most upgrade scripts will
have to be rewritten and/or moved to other packages.