bootm: Support android boot on sandbox

A small change allows this to operate on sandbox.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/configs/sandbox.h b/include/configs/sandbox.h
index 6bb2546..a145094 100644
--- a/include/configs/sandbox.h
+++ b/include/configs/sandbox.h
@@ -41,6 +41,7 @@
 #define CONFIG_RSA
 #define CONFIG_CMD_FDT
 #define CONFIG_DEFAULT_DEVICE_TREE	sandbox
+#define CONFIG_ANDROID_BOOT_IMAGE
 
 #define CONFIG_FS_FAT
 #define CONFIG_FS_EXT4