base-files: allow ignoring minor compat-version check

base-files: allow ignoring minor compat-version check

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>
(cherry picked from commit 34437af88867c4435add8a144417290b7fd4362a)