Downstream projects might re-generate device-specific configuration
based on OpenWrt's defaults on each upgrade, thus being unaffected by
forward- as well as backwards-breaking configuration.
Add a new sysupgrade parameter, which allows sysupgrades between minor
compat-versions. Upgrades will still fail upon mismatching major compat
versions.
Signed-off-by: David Bauer <mail@david-bauer.net>