x86: Exit EFI boot services before starting kernel

x86: Exit EFI boot services before starting kernel

When running the EFI app, we need to exit boot services before jumping
to Linux.

At some point it may be possible to jump to Linux and pass on the system
table, and:

  * install the device-tree as configuration table
  * use LoadImage() to load the kernel image (e.g. from memory)
  * start the image with StartImage()

This should allow the Linux efistub to be used. For now, this is not
implemented.

Signed-off-by: Simon Glass <sjg@chromium.org>