Convert CONFIG_ENV_IS_IN_SPI_FLASH to Kconfig

This converts the following to Kconfig:
   CONFIG_ENV_IS_IN_SPI_FLASH

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/marsboard_defconfig b/configs/marsboard_defconfig
index b6fbbb9..080815d 100644
--- a/configs/marsboard_defconfig
+++ b/configs/marsboard_defconfig
@@ -3,7 +3,8 @@
 CONFIG_TARGET_EMBESTMX6BOARDS=y
 CONFIG_VIDEO=y
 CONFIG_DISTRO_DEFAULTS=y
-CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=board/boundary/nitrogen6x/nitrogen6q.cfg,MX6Q,DDR_MB=1024,ENV_IS_IN_SPI_FLASH"
+CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=board/boundary/nitrogen6x/nitrogen6q.cfg,MX6Q,DDR_MB=1024"
+CONFIG_ENV_IS_IN_SPI_FLASH=y
 # CONFIG_CONSOLE_MUX is not set
 CONFIG_SYS_CONSOLE_IS_IN_ENV=y
 CONFIG_SYS_CONSOLE_OVERWRITE_ROUTINE=y