x86: Allow excluding reset vector code from u-boot

When running from coreboot we don't want this code.

This version works by ifdef-ing out all of the code that would go
into those sections and all the code that refers to it. The sections are
then empty, and the linker will either leave them empty for the loader
to ignore or remove them entirely.

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