x86: Support writing configuration tables in high area

For those secondary bootloaders like SeaBIOS who want to live in
the F segment, which conflicts the configuration table address,
now we allow write_tables() to write the configuration tables in
high area (malloc'ed memory).

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>