SPL: Add CONFIG_SPL_ABORT_ON_RAW_IMAGE

When defined, SPL will proceed to another boot method
if the image it has loaded does not have a signature.
This is useful if the subsequent boot methods are much
more complex.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Tom Rini <trini@konsulko.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Peng Fan <van.freenix@gmail.com>
Cc: Fabio Estevam <fabio.estevam@nxp.com>

README

@@ -3487,6 +3487,10 @@ FIT uImage format:
 		consider that a completely unreadable NAND block is bad,
 		and thus should be skipped silently.
 
+		CONFIG_SPL_ABORT_ON_RAW_IMAGE
+		When defined, SPL will proceed to another boot method
+		if the image it has loaded does not have a signature.
+
 		CONFIG_SPL_RELOC_STACK
 		Adress of the start of the stack SPL will use after
 		relocation.  If unspecified, this is equal to

common/spl/spl.c

@@ -111,6 +111,9 @@ int spl_parse_image_header(const struct image_header *header)
 		 * is bad, and thus should be skipped silently.
 		 */
 		panic("** no mkimage signature but raw image not supported");
+#elif defined(CONFIG_SPL_ABORT_ON_RAW_IMAGE)
+		/* Signature not found, proceed to other boot methods. */
+		return -EINVAL;
 #else
 		/* Signature not found - assume u-boot.bin */
 		debug("mkimage signature not found - ih_magic = %x\n",