Fix a "scan_dev_for_efi" not defined error
    
    A previous commit dropped a NUL character at the end of scan_dev_for_dtb
    definition and this caused scan_dev_for_efi to not be defined. Following
    error is shown by U-Boot when trying to boot an EFI binary:
    
     Scanning mmc 0:2...
     ## Error: "scan_dev_for_efi" not defined