x86: edison: Switch to CONFIG_OF_SEPARATE

There is no need for Intel Edison to have CONFIG_OF_EMBED to be enabled.
Replace it with CONFIG_OF_SEPARATE.

There is no functional change since u-boot.bin always contains DTB
either embedded or attached.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Ferry Toth <ftoth@exalondelft.nl>