spl_mmc.c: Detect missing kernel image in RAW MMC

Currently, we assume that if we can read from MMC correctly, we have
found a valid image.  This is not the case as an empty area will read
just fine.  Add a check for a valid IH_MAGIC.
Signed-off-by: NTom Rini <trini@ti.com>
Reviewed-by: NPeter Korsgaard <jacmet@sunsite.dk>

common/spl/spl_mmc.c

@@ -13,6 +13,7 @@
 #include <mmc.h>
 #include <fat.h>
 #include <version.h>
+#include <image.h>
 
 DECLARE_GLOBAL_DATA_PTR;
 
@@ -30,6 +31,9 @@ static int mmc_load_image_raw(struct mmc *mmc, unsigned long sector)
 	if (err == 0)
 		goto end;
 
+	if (image_get_magic(header) != IH_MAGIC)
+		return -1;
+
 	spl_parse_image_header(header);
 
 	/* convert size to sectors - round up */