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/snow_defconfig b/configs/snow_defconfig
index 5cccd91..5a197d1 100644
--- a/configs/snow_defconfig
+++ b/configs/snow_defconfig
@@ -10,6 +10,7 @@
 CONFIG_DISTRO_DEFAULTS=y
 CONFIG_FIT=y
 CONFIG_FIT_BEST_MATCH=y
+CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_SILENT_CONSOLE=y
 CONFIG_SPL=y
 CONFIG_SYS_PROMPT="snow # "