Fix fwpath in efi netboot
    
    The path returned by grub_efi_net_config has already been stripped for the
    directory part extracted from cached bootp packet. We should just return the
    result to avoild it be stripped again.
    
    It fixed the problem that grub.efi as NBP image always looking for grub.cfg and
    platform directory in upper folder rather than current one it gets loaded while
    $prefix is empty. The behavior is inconsistent with other architecture and how
    we would expect empty $prefix going to be in general.
    
    The only exception to the general rule of empty $prefix is that when loaded
    from platform directory itself, the platform part is stripped thus upper folder
    is used for looking up files. It meets the case for how grub-mknetdir lay out
    the files under tftp root directory, but also hide away this issue to be
    identified as it appears to be just works.
    
    Also fix possible memory leak by moving grub_efi_get_filename() call after
    grub_efi_net_config().