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/dms-ba16_defconfig b/configs/dms-ba16_defconfig
index 7a8c268..dd3428e 100644
--- a/configs/dms-ba16_defconfig
+++ b/configs/dms-ba16_defconfig
@@ -2,6 +2,7 @@
 CONFIG_ARCH_MX6=y
 CONFIG_TARGET_ADVANTECH_DMS_BA16=y
 CONFIG_VIDEO=y
+CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_BOOTDELAY=1
 # CONFIG_CONSOLE_MUX is not set
 CONFIG_SYS_CONSOLE_IS_IN_ENV=y