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/smdk5250_defconfig b/configs/smdk5250_defconfig
index de62cb3..4189475 100644
--- a/configs/smdk5250_defconfig
+++ b/configs/smdk5250_defconfig
@@ -7,6 +7,7 @@
 CONFIG_FIT=y
 CONFIG_FIT_BEST_MATCH=y
 CONFIG_SILENT_CONSOLE=y
+CONFIG_CONSOLE_MUX=y
 CONFIG_SPL=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="SMDK5250 # "