x86: Add an option to control the position of U-Boot

x86: Add an option to control the position of U-Boot

The existing work-around for positioning U-Boot in the ROM when it
actually runs from RAM still exists and there is not obvious way to change
this.

Add a proper Kconfig option to handle this case. This also adds a new bool
property to indicate whether CONFIG_SYS_TEXT_BASE exists.

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