sandbox: Enable more console options

Enable the pre-console buffer, displaying the model and post-relocation
console announce on sandbox. Also add a model name to the device tree.
This allows testing of these features.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Stephen Warren <swarren@nvidia.com>

arch/sandbox/dts/sandbox.dts

@@ -5,6 +5,7 @@
 / {
 	#address-cells = <1>;
 	#size-cells = <1>;
+	model = "sandbox";
 
 	aliases {
 		eth5 = "/eth@90000000";