flatcar-install: make sure blkid returns one device

[ader1990]

In some cases where path disk devices have multiple copies, blkid should return only one device for label.

According to https://linux.die.net/man/8/blkid, the -l would be perfect for this, as it returns the regular block device first.

Fixes: flatcar/Flatcar#1506

[krnowak]

man blkid for --list-one says as follows:

Look up only one device that matches the search parameter specified with the --match-token option. If there are multiple devices that match the specified search parameter, then the device with the highest priority is returned, and/or the first device found at a given priority (but see below note about udev). Device types in order of decreasing priority are: Device Mapper, EVMS, LVM, MD, and finally regular block devices. If this option is not specified, blkid will print all of the devices that match the search parameter.

So looks like regular block device is of lowest priority. I'm not sure if it matters, though - we still want to have just one device, so this seems to be an alright change. What else we could do is to slap | head -n1 instead, but this is as justified as your -l and arguably hackier. I'm inclined to approve it.

[ader1990]

Thanks @krnowak! I will try to look for another alternative this week, and if I do not find a more suitable approach, I will merge it.