efi_loader: refactor boot device and loaded_image handling

Get rid of the hacky fake boot-device and duplicate device-path
constructing (which needs to match what efi_disk and efi_net do).
Instead convert over to use efi_device_path helpers to construct
device-paths, and use that to look up the actual boot device.

Also, extract out a helper to plug things in properly to the
loaded_image.  In a following patch we'll want to re-use this in
efi_load_image() to handle the case of loading an image from a
file_path.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Alexander Graf <agraf@suse.de>