scripts: size_compare: print a grand total

Usefull to check the impact of treewide changes:
Change 	Local	Remote 	Package
+281	6191	5910	ubus
-547	56166	56713	procd
-13294	91544	104838	ubi-utils
~~~~~~~	total change	-13560

Signed-off-by: Andre Heider <a.heider@gmail.com>

scripts/size_compare.sh

@@ -75,6 +75,7 @@ package_size () {
 }
 
 compare_sizes () {
+	TOTAL_DIFF="0"
 	for PACKAGE in $PACKAGES; do
 		if [ "$PACKAGE" = "libc" ]; then
 			continue
@@ -93,6 +94,7 @@ compare_sizes () {
 		fi
 		SIZE_UPSTREAM=$(package_size "$TMP_INDEX" "$PACKAGE")
 		SIZE_DIFF="$((SIZE_LOCAL - SIZE_UPSTREAM))"
+		TOTAL_DIFF="$((TOTAL_DIFF + SIZE_DIFF))"
 		if [ "$SIZE_DIFF" -gt 0 ]; then
 			SIZE_DIFF="+$SIZE_DIFF"
 		fi
@@ -102,6 +104,7 @@ compare_sizes () {
 			echo "$PACKAGE is missing upstream"
 		fi
 	done
+	echo "~~~~~~~	total change	${TOTAL_DIFF}"
 }
 
 if [ "$1" = "-h" ]; then