Use lazy umount to avoid race problems with dev

A problem with unmounting the dev filesystem in Ubuntu images caused
the umount of the /dev bind mount to fail, which left it there to be
removed during the mv -t step, causing the build host's /dev to be
wiped out.

The lazy umount will detach it from the filesystem hierarchy and then
clean up the mount reference later.

Change-Id: I8f8cea857c445fb0b4fd02bc063722fb1553c947