kirkwood: implement compatibility version for DSA migration

kirkwood: implement compatibility version for DSA migration

This implements the newly introduced compat-version to prevent
upgrade between swconfig and DSA for kirkwood.

Just define a compat version with minor increment and an appropriate
message for both image (in Makefile) and device (in base-files).

Since we never removed SUPPORTED_DEVICES for this target, we don't
have to add it back either.

Attention:

All users that already updated to the DSA versions in master will
receive the same incompatibility warning since their devices are still
"1.0" as far as fwtool can tell.
Those, and only those, can bypass the upgrade check by using force (-F)
without having to reset config again. In addition, the new version
string needs to be put into uci config manually, so the new fwtool
knows that it actually deals with a "1.1":

   uci set "system.@system[-1].compat_version=1.1"
   uci commit system

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