SECURE_BOOT: Enable SD as a source for bootscript

Add support for reading bootscript and bootscript header from SD. Also
renamed macros *_FLASH to *_DEVICE to represent SD alongwith NAND and
NOR flash.

Reviewed-by: Aneesh Bansal <aneesh.bansal@nxp.com>
Signed-off-by: Sumit Garg <sumit.garg@nxp.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: York Sun <york.sun@nxp.com>