efi_loader: correctly call images

Avoid a failed assertion when an EFI app calls an EFI app.

Avoid that the indent level increases when calling 'bootefi hello'
repeatedly.

Avoid negative indent level when an EFI app calls an EFI app that
calls an EFI app (e.g. iPXE loads grub which starts the kernel).

Return the status code of a loaded image that returns without
calling the Exit boot service.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>