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/smdk5420_defconfig b/configs/smdk5420_defconfig
index bff9444..6a2ea20 100644
--- a/configs/smdk5420_defconfig
+++ b/configs/smdk5420_defconfig
@@ -7,6 +7,7 @@
 CONFIG_DISTRO_DEFAULTS=y
 CONFIG_FIT=y
 CONFIG_FIT_BEST_MATCH=y
+CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_SILENT_CONSOLE=y
 CONFIG_CONSOLE_MUX=y
 CONFIG_SPL=y