efi_loader: check initialization of EFI subsystem is successful

Up to now errors in the initialization of the EFI subsystems was not
checked.

If any initialization fails, leave the bootefi command.

We do not retry initialization because this would require to undo all prior
initalization steps.

Suggested-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>