luksroot: Wait for the header (device) to appear

The LUKS header can be on another device (e.g. a USB stick). In my case
it can take up to two seconds until the partition on my USB stick is
available (i.e. the decryption fails without this patch). This will also
remove some redundancy by providing the shell function `wait_target` and
slightly improve the output (one "." per second and a success/failure
indication after 10 seconds instead of always printing "ok").