ramips: mt7621: implement compatibility version for DSA migration

ramips: mt7621: implement compatibility version for DSA migration

This implements the newly introduced compat-version to prevent
broken upgrade between swconfig and DSA for ramips' mt7621 subtarget.

In order to make the situation more transparent for the user, and
to prevent large switch-cases for devices, it is more convenient to
have the entire subtarget 1.1-by-default. This means that new devices
will be added with 1.1 from the start, but in contrast we don't need
to switch them in board.d files. Apart from that, users that manually
backport devices to 19.07 with swconfig will have an equivalent
upgrade experience to officially supported devices.

Since DSA support on mt7621 is out for a while already, this applies
the same uci-defaults workaround for early adopters as already
done for kirkwood and mvebu in previous commits.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>