bootstd: Add a sandbox bootmeth driver

Add a bootmeth driver for sandbox, used for testing.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/boot/Kconfig b/boot/Kconfig
index 0bfea27..373871c 100644
--- a/boot/Kconfig
+++ b/boot/Kconfig
@@ -370,6 +370,13 @@
 
 	  This provides a way to try out standard boot on an existing boot flow.
 
+config BOOTMETH_SANDBOX
+	def_bool y
+	depends on SANDBOX
+	help
+	  This is a sandbox bootmeth driver used for testing. It always returns
+	  -ENOTSUPP when attempting to boot.
+
 endif
 
 config LEGACY_IMAGE_FORMAT