arm: mvebu: clearfog: add scsi target to distro-boot

Support for sata devices via the scsi command is available and already
enabled by default for the Clearfog Base and Pro. This change adds scsi
to the list of boot targets used by distro-boot.

Signed-off-by: Josua Mayer <josua@solid-run.com>
Cc: Stefan Roese <sr@denx.de>
Reviewed-by: Stefan Roese <sr@denx.de>