x86: Convert microcode format to device-tree-only

To avoid having two microcode formats, adjust the build system to support
obtaining the microcode from the device tree, even in the case where it
must be made available before the device tree can be accessed.

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