x86: Allow setup code to manage its own global data

Currently x86 has its own means of managing the global data and board data
(bd_t), and this code resides in start.S. With generic board, we need to
ensure that we leave this alone - i.e. don't clear it as we do on other
archs.

This fixes a problem where the memory init data is cleared which causes
the video driver to operate very slowly.

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