sandbox: Move CONFIG_SANDBOX_SERIAL to Kconfig

Move this over to Kconfig and tidy up.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/serial/Kconfig b/drivers/serial/Kconfig
index 5611fac..4829284 100644
--- a/drivers/serial/Kconfig
+++ b/drivers/serial/Kconfig
@@ -76,6 +76,26 @@
 	  value. Use this value to specify the shift to use, where 0=byte
 	  registers, 2=32-bit word registers, etc.
 
+config SANDBOX_SERIAL
+	bool "Sandbox UART support"
+	depends on SANDBOX && DM
+	help
+	  Select this to enable a seral UART for sandbox. This is required to
+	  operate correctly, otherwise you will see no serial output from
+	  sandbox. The emulated UART will display to the console and console
+	  input will be fed into the UART. This allows you to interact with
+	  U-Boot.
+
+	  The operation of the console is controlled by the -t command-line
+	  flag. In raw mode, U-Boot sees all characters from the terminal
+	  before they are processed, including Ctrl-C. In cooked mode, Ctrl-C
+	  is processed by the terminal, and terminates U-Boot. Valid options
+	  are:
+
+	     -t raw-with-sigs	Raw mode, Ctrl-C will terminate U-Boot
+	     -t raw		Raw mode, Ctrl-C is processed by U-Boot
+	     -t cooked		Cooked mode, Ctrl-C terminates
+
 config UNIPHIER_SERIAL
 	bool "Support for UniPhier on-chip UART"
 	depends on ARCH_UNIPHIER && DM_SERIAL