efi_loader: LoadImage() check parent image

If the parent image handle does not refer to a loaded image return
EFI_INVALID_PARAMETER.
(UEFI SCT II 2017: 3.4.1 LoadImage() - 5.1.4.1.1)

Mark our root node as a loaded image to avoid an error when using it as
parent image.

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