efi_loader: Pass file path to payload

The payload gets information on where it got loaded from. This includes
the device as well as file path.

So far we've treated both as the same thing and always gave it the device
name. However, in some situations grub2 actually wants to find its loading
path to find its configuration file.

So let's split the two semantically separte bits into separate structs and
pass the loaded file name into our payload when we load it using "load".

Signed-off-by: Alexander Graf <agraf@suse.de>