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>
diff --git a/README b/README
index 88ff837..d881da2 100644
--- a/README
+++ b/README
@@ -3487,6 +3487,10 @@
 		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