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/MPC8536DS_SPIFLASH_defconfig b/configs/MPC8536DS_SPIFLASH_defconfig
index f161b61..3ec85ac 100644
--- a/configs/MPC8536DS_SPIFLASH_defconfig
+++ b/configs/MPC8536DS_SPIFLASH_defconfig
@@ -6,6 +6,7 @@
 CONFIG_OF_STDOUT_VIA_ALIAS=y
 CONFIG_SYS_EXTRA_OPTIONS="SPIFLASH"
 CONFIG_BOOTDELAY=10
+# CONFIG_CONSOLE_MUX is not set
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_MMC=y
 CONFIG_CMD_SF=y