rockchip: configs: sandbox: enable rkmtd command

Enable rkmtd command for testing with sandbox_defconfig
and sandbox64_defconfig.

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/sandbox64_defconfig b/configs/sandbox64_defconfig
index 1a033b2..1a01f51 100644
--- a/configs/sandbox64_defconfig
+++ b/configs/sandbox64_defconfig
@@ -58,6 +58,7 @@
 CONFIG_CMD_SPI=y
 CONFIG_CMD_TEMPERATURE=y
 CONFIG_CMD_USB=y
+CONFIG_CMD_RKMTD=y
 CONFIG_CMD_WDT=y
 CONFIG_CMD_WRITE=y
 CONFIG_CMD_CAT=y