[efi] Wrap any images loaded by our wrapped image

Propagate our modified EFI system table to any images loaded by the
image that we wrap, thereby allowing us to observe boot services calls
made by all subsequent EFI images.

Also show details of intercepted ExitBootServices() calls.  When
wrapping is used, exiting boot services will almost certainly fail,
but this at least allows us to see when it happens.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
Merge remote-tracking branch 'upstream/master'