sandbox: Add serial uart

This uart simply writes to stdout and reads from stdin. We might imagine
instead buffering the data so that a test interface can check output and
inject input.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/serial/Makefile b/drivers/serial/Makefile
index 1dcc1c7..6309549 100644
--- a/drivers/serial/Makefile
+++ b/drivers/serial/Makefile
@@ -53,6 +53,7 @@
 COBJS-$(CONFIG_S3C24X0_SERIAL) += serial_s3c24x0.o
 COBJS-$(CONFIG_S3C44B0_SERIAL) += serial_s3c44b0.o
 COBJS-$(CONFIG_XILINX_UARTLITE) += serial_xuartlite.o
+COBJS-$(CONFIG_SANDBOX_SERIAL) += sandbox.o
 COBJS-$(CONFIG_SCIF_CONSOLE) += serial_sh.o
 COBJS-$(CONFIG_TEGRA2) += serial_tegra2.o