hob: sync after image deploy

Showing "Deploy image successful" after 'dd' returns
may determine the user to disconnect the usb stick even
though the writing operations are not finished.
This patch makes sure that the entire image is deployed
on the usb stick before the user is informed about any result.

[YOCTO #5892]

Signed-off-by: Valentin Popa <valentin.popa@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>