treewide: sysupgrade: pass "save_partitions" option to the "sysupgrade" method

This explicitly lets stage2 know if partitions should be preserved. No
more "touch /tmp/sysupgrade.always.overwrite.bootdisk.partmap" hack.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
(cherry picked from commit b6f4cd57e1)

target/linux/brcm2708/base-files/lib/upgrade/platform.sh

@@ -45,7 +45,7 @@ platform_do_upgrade() {
 
 	sync
 
-	if [ "$SAVE_PARTITIONS" = "1" ]; then
+	if [ "$UPGRADE_OPT_SAVE_PARTITIONS" = "1" ]; then
 		get_partitions "/dev/$diskdev" bootdisk
 
 		#extract the boot sector from the image