mkimage: Add OMAP boot image support

- Add mkimage support for OMAP boot image
- Add support for OMAP boot image(MLO) generation in the new
  SPL framework

Signed-off-by: John Rigby <john.rigby@linaro.org>
Signed-off-by: Aneesh V <aneesh@ti.com>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>