x86: Create a new header for EFI

The setup routines are called from zimage but don't really belong in the
zimage header. Add a new EFI header to house these. Add comments so it is
clear what the functions do.

Note that these functions are x86-specific. The zimage business is not
used on other architectures.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>