efidebug: avoid 'dfu_alt_info not defined' message

efidebug: avoid 'dfu_alt_info not defined' message

If variable dfu_alt_info is not defined duplicate messages are displayed.

    => efidebug boot dump
    Scanning disk mmc2.blk...
    Scanning disk mmc1.blk...
    Scanning disk mmc0.blk...
    Found 3 disks
    No EFI system partition
    "dfu_alt_info" env variable not defined!
    Probably dfu_alt_info not defined
    "dfu_alt_info" env variable not defined!
    Probably dfu_alt_info not defined

Remove the 'Probably dfu_alt_info not defined' message.
Instead write a warning if the variable contains no entities.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>