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>