payloads/Kconfig: Add NO_DEFAULT_PAYLOAD

This symbol can be selected (e.g. in site-local/) to disable the user
friendly but annoying default payload choice.

Change-Id: I2f72d4efc0a428dce377c3d003b2c00a6c8d4c08
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/19808
Reviewed-by: Philippe Mathieu-Daudé <philippe.mathieu.daude@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin Roth <martinroth@google.com>

payloads/Kconfig

@@ -1,8 +1,11 @@
 menu "Payload"
 
+config NO_DEFAULT_PAYLOAD
+	bool
+
 choice
 	prompt "Add a payload"
-	default PAYLOAD_NONE if !ARCH_X86
+	default PAYLOAD_NONE if NO_DEFAULT_PAYLOAD || !ARCH_X86
 	default PAYLOAD_SEABIOS if ARCH_X86
 
 config PAYLOAD_NONE