efi_loader: Add boot time services

When an EFI application runs, it has access to a few descriptor and callback
tables to instruct the EFI compliant firmware to do things for it. The bulk
of those interfaces are "boot time services". They handle all object management,
and memory allocation.

This patch adds support for the boot time services and also exposes a system
table, which is the point of entry descriptor table for EFI payloads.

Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>