efi_loader: create boot options without file path

Allow the efidebug command to create boot options without file path, e.g.

    efidebug boot add -b 0001 'short dev only' host 0:1 ''
    efidebug boot add -B 0002 'long dev only' host 0:1 ''

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>