base-files: sysupgrade: do not rely on opkg to list changed conffiles

Many packages use the opkg conffiles field to list configuration files that
are to be retained on upgrades. Make this work on systems without opkg.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>

package/base-files/files/sbin/sysupgrade

@@ -96,12 +96,31 @@ EOF
 # prevent messages from clobbering the tarball when using stdout
 [ "$CONF_BACKUP" = "-" ] && export VERBOSE=0
 
+
+list_conffiles() {
+	awk '
+		BEGIN { conffiles = 0 }
+		/^Conffiles:/ { conffiles = 1; next }
+		!/^ / { conffiles = 0; next }
+		conffiles == 1 { print }
+	' /usr/lib/opkg/status
+}
+
+list_changed_conffiles() {
+	# Cannot handle spaces in filenames - but opkg cannot either...
+	list_conffiles | while read file csum; do
+		[ -r "$file" ] || continue
+
+		echo "${csum}  ${file}" | sha256sum -sc - || echo "$file"
+	done
+}
+
 add_uci_conffiles() {
 	local file="$1"
 	( find $(sed -ne '/^[[:space:]]*$/d; /^#/d; p' \
 		/etc/sysupgrade.conf /lib/upgrade/keep.d/* 2>/dev/null) \
 		-type f -o -type l 2>/dev/null;
-	  opkg list-changed-conffiles ) | sort -u > "$file"
+	  list_changed_conffiles ) | sort -u > "$file"
 	return 0
 }