soc/intel/skylake/Kconfig: Fix recursive Kconfig dependency

Change the dependency on CONSOLE_SERIAL to select CONSOLE_SERIAL based
on this question.
The dependency was causing multiple warnings on every platform tested.

src/console/Kconfig:21:error: recursive dependency detected!
src/console/Kconfig:21:	symbol CONSOLE_SERIAL depends on
DRIVERS_UART_8250MEM
src/drivers/uart/Kconfig:16:	symbol DRIVERS_UART_8250MEM is selected by
UART_DEBUG
src/soc/intel/skylake/Kconfig:198:	symbol UART_DEBUG depends on
CONSOLE_SERIAL

Change-Id: Ia0426cd150561694081b5ea7c6797d36022c1f57
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: http://review.coreboot.org/11243
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Tested-by: build bot (Jenkins)
diff --git a/src/soc/intel/skylake/Kconfig b/src/soc/intel/skylake/Kconfig
index 6c64cd9..9d920d8 100644
--- a/src/soc/intel/skylake/Kconfig
+++ b/src/soc/intel/skylake/Kconfig
@@ -195,8 +195,8 @@
 
 config UART_DEBUG
 	bool "Enable UART debug port."
-	default y if CONSOLE_SERIAL
 	default n
+	select CONSOLE_SERIAL
 	select DRIVERS_UART
 	select DRIVERS_UART_8250MEM
 	select DRIVERS_UART_8250MEM_32