Convert esdhc_start_image() to use constants and data types from
<mach/imx-header.h>. Also, while at it, define a simple inline
function to test if an arbitrary binary blob is i.MX flash header v2.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>