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/aristainetos_defconfig b/configs/aristainetos_defconfig
index 0a8b38d..525aff4 100644
--- a/configs/aristainetos_defconfig
+++ b/configs/aristainetos_defconfig
@@ -4,6 +4,7 @@
 CONFIG_VIDEO=y
 CONFIG_FIT=y
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=board/aristainetos/aristainetos.cfg,MX6DL"
+CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_BOOTDELAY=3
 # CONFIG_CONSOLE_MUX is not set
 CONFIG_SYS_CONSOLE_IS_IN_ENV=y