Convert CONFIG_SYS_CONSOLE_IS_IN_ENV and CONFIG_CONSOLE_MUX to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_CONSOLE_IS_IN_ENV
   CONFIG_CONSOLE_MUX

Signed-off-by: Simon Glass <sjg@chromium.org>
[trini: Re-order, re-migrate]
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/riotboard_defconfig b/configs/riotboard_defconfig
index 404d25e..fe522c0 100644
--- a/configs/riotboard_defconfig
+++ b/configs/riotboard_defconfig
@@ -3,6 +3,8 @@
 CONFIG_TARGET_EMBESTMX6BOARDS=y
 CONFIG_VIDEO=y
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=board/boundary/nitrogen6x/nitrogen6s1g.cfg,MX6S,DDR_MB=1024,ENV_IS_IN_MMC"
+# CONFIG_CONSOLE_MUX is not set
+CONFIG_SYS_CONSOLE_IS_IN_ENV=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMLS is not set