efi.h: Add struct definition for boot time services

With the forthcoming efi stub code we're gonna need to access boot
time services so let's define a struct so we can access the functions.

Cc: Matthew Garrett <mjg@redhat.com>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>
Link: http://lkml.kernel.org/r/1318848017-12301-1-git-send-email-matt@console-pimps.org


Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>