efi_loader: Make exposed image loader path absolute

When loading an efi image, we pass it the location it was loaded from.

On file system backends, there are no relative paths, so we should always
pass in absolute ones. For network paths, we may be relative.

This fixes distro booting with grub2 for me when it fetches the grub2 config
file from the loader partition.

Reported-by: york sun <york.sun@nxp.com>
Signed-off-by: Alexander Graf <agraf@suse.de>