x86: edison: Generate an image suitable for xFSTK

It is useful to be able to flash Edison directly without relying on the
installed U-Boot being functional.

Add a binman image for this. It includes a 'OSIP' header (which happens to
look like an MBR / (Master-Boot Record), U-Boot binary and an environment.

I am not able to find a specification for OSIP.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>