sysupgrade: unmount filesystems before reboot

sysupgrade immediately reboots after flashing an image and doesn't
allow to unmount filesystems. At least in case the image used for
sysupgrade is stored on a FAT formatted usb flash drive, the following
warning is printed during the next mount of the flash drive:

FAT-fs (sda1): Volume was not properly unmounted. Some data may be
corrupt. Please run fsck.

Although a data corruption during read operations is unlikely, there is
no need to scare the users.

Signed-off-by: Mathias Kresin <dev@kresin.me>

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

@@ -243,6 +243,7 @@ do_upgrade() {
 	[ -n "$DELAY" ] && sleep "$DELAY"
 	ask_bool 1 "Reboot" && {
 		v "Rebooting system..."
+		umount -a
 		reboot -f
 		sleep 5
 		echo b 2>/dev/null >/proc/sysrq-trigger

package/system/procd/files/nand.sh

@@ -198,6 +198,7 @@ nand_do_upgrade_success() {
 	sync
 	[ -f "$conf_tar" ] && nand_restore_config "$conf_tar"
 	echo "sysupgrade successful"
+	umount -a
 	reboot -f
 }
 
@@ -213,6 +214,7 @@ nand_upgrade_ubinized() {
 
 	if [ ! "$mtdnum" ]; then
 		echo "cannot find mtd device $CI_UBIPART"
+		umount -a
 		reboot -f
 	fi