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 34437af888)

package/base-files/files/lib/upgrade/fwtool.sh

@@ -71,6 +71,7 @@ fwtool_check_image() {
 
 			# minor compat version -> sysupgrade with -n required
 			if [ "${devicecompat#.*}" != "${imagecompat#.*}" ] && [ "$SAVE_CONFIG" = "1" ]; then
+				[ "$IGNORE_MINOR_COMPAT" = 1 ] && return 0
 				v "The device is supported, but the config is incompatible to the new image ($devicecompat->$imagecompat). Please upgrade without keeping config (sysupgrade -n)."
 				[ -n "$compatmessage" ] && v "$compatmessage"
 				return 1

package/base-files/files/sbin/sysupgrade

@@ -19,6 +19,7 @@ export CONF_IMAGE=
 export CONF_BACKUP_LIST=0
 export CONF_BACKUP=
 export CONF_RESTORE=
+export IGNORE_MINOR_COMPAT=0
 export NEED_IMAGE=
 export HELP=0
 export FORCE=0
@@ -44,6 +45,7 @@ while [ -n "$1" ]; do
 		-F|--force) export FORCE=1;;
 		-T|--test) export TEST=1;;
 		-h|--help) export HELP=1; break;;
+		--ignore-minor-compat-version) export IGNORE_MINOR_COMPAT=1;;
 		-*)
 			echo "Invalid option: $1" >&2
 			exit 1
@@ -80,6 +82,8 @@ upgrade-option:
 	             Verify image and config .tar.gz but do not actually flash.
 	-F | --force
 	             Flash image even if image checks fail, this is dangerous!
+	--ignore-minor-compat-version
+		     Flash image even if the minor compat version is incompatible.
 	-q           less verbose
 	-v           more verbose
 	-h | --help  display this help