splash_source: Verify FIT magic

Before reading entire FIT image, add sanity check by testing image
header against FDT_MAGIC. This should help avoid problems in situations
where FIT is not yet available from storage device, for example when
performing initial programming of device.

Cc: Anatolij Gustschin <agust@denx.de>
Acked-by: Tomas Melin <tomas.melin@vaisala.com>