Pessimize usb devices when determining boot device

Otherwise it believes that this machine is booted via floppy:

pci-0000:00:1a.0-usb-0:1.4:1.0-scsi-0:0:0:0 -> ../../sr0
pci-0000:00:1a.0-usb-0:1.4:1.0-scsi-0:0:0:1 -> ../../sde
pci-0000:00:1a.0-usb-0:1.4:1.0-scsi-0:0:0:2 -> ../../sdf
pci-0000:00:1f.2-scsi-1:0:0:0 -> ../../sda
pci-0000:00:1f.2-scsi-2:0:0:0 -> ../../sdb
pci-0000:00:1f.2-scsi-3:0:0:0 -> ../../sdc
pci-0000:00:1f.2-scsi-4:0:0:0 -> ../../sdd

Change it into a two pass approach, first ignoring usb, then going over
all devices.
Merge pull request #245 from dirkmueller/release/roxy/master

Pessimize usb devices when determining boot device