sandbox: Add architecture image support

We won't actually load an image with this architecture, but we still need to
define it.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/include/asm/u-boot.h b/arch/sandbox/include/asm/u-boot.h
index 7d91847..166ef14 100644
--- a/arch/sandbox/include/asm/u-boot.h
+++ b/arch/sandbox/include/asm/u-boot.h
@@ -58,4 +58,7 @@
 	} bi_dram[CONFIG_NR_DRAM_BANKS];
 } bd_t;
 
+/* For image.h:image_check_target_arch() */
+#define IH_ARCH_DEFAULT IH_ARCH_SANDBOX
+
 #endif	/* _U_BOOT_H_ */