Kconfig: Recommend not enabling UPDATE_IMAGE option

We just had a user who spent a fair amount of time debugging a
failing build due to this option being enabled.  Add a little
guidance that it probably shouldn't be enabled in the help text.

Change-Id: I9339e442876c1fcd18ea564041c6cc1201c18ae5
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/13066
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

src/Kconfig

@@ -309,6 +309,8 @@ config UPDATE_IMAGE
 	  is a suitable file for further processing.
 	  The bootblock will not be modified.
 
+	  If unsure, select 'N'
+
 config GENERIC_GPIO_LIB
 	bool
 	default n