efi_loader: allow booting from short dev only DP

Allow booting from a short form device-path without file path, e.g.

    /HD(1,GPT,5ef79931-a1aa-4c70-9d67-611e8f69eafd,0x800,0x1000)

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