Convert CONFIG_ENV_IS_IN_FLASH to Kconfig

This converts the following to Kconfig:
   CONFIG_ENV_IS_IN_FLASH

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/mccmon6_nor_defconfig b/configs/mccmon6_nor_defconfig
index b0f2692..449804b 100644
--- a/configs/mccmon6_nor_defconfig
+++ b/configs/mccmon6_nor_defconfig
@@ -6,6 +6,7 @@
 CONFIG_SPL_SERIAL_SUPPORT=y
 # CONFIG_CMD_BMODE is not set
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=board/liebherr/mccmon6/mon6_imximage_nor.cfg,MX6QDL"
+CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_SPL=y
 CONFIG_SPL_BOARD_INIT=y
 CONFIG_SPL_ENV_SUPPORT=y