imx6: apalis: fix config survival after sysupgrade -n

This patch fixes `sysupgrade -n` when flashed with rootfs of the same
size as currently running, so the rootfs_data wouldn't get destroyed and
thus survive reboot. So let's fix it by always cleaning up the content
of the rootfs_data during sysupgrade.

Signed-off-by: Petr Štetiar <ynezz@true.cz>

target/linux/imx6/base-files/lib/upgrade/platform.sh

@@ -72,3 +72,16 @@ platform_copy_config() {
 		;;
 	esac
 }
+
+platform_pre_upgrade() {
+	local board=$(board_name)
+
+	case "$board" in
+	apalis*)
+		[ "$SAVE_CONFIG" -eq 0 ] && {
+			jffs2reset -y
+			umount /overlay
+		}
+		;;
+	esac
+}