There was a race in diskimage-builder where the mkfs call after a
kpartx -avs for the loop device would fail because the device was
not yet ready. This adds a udevadm settle call after the kpartx
to make sure the udev event queue has cleared.
Change-Id: I90103b59357edebbac7a641e8980cb282d37561b
Closes-Bug: #1698337