Convert CONFIG_ENV_IS_IN_FLASH to Kconfig

This converts the following to Kconfig:
   CONFIG_ENV_IS_IN_FLASH

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/adp-ae3xx_defconfig b/configs/adp-ae3xx_defconfig
index d47ad86..9e1e5ec 100644
--- a/configs/adp-ae3xx_defconfig
+++ b/configs/adp-ae3xx_defconfig
@@ -13,6 +13,7 @@
 CONFIG_CMD_EXT2=y
 CONFIG_CMD_FAT=y
 CONFIG_OF_CONTROL=y
+CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_NET_RANDOM_ETHADDR=y
 CONFIG_DM=y
 CONFIG_MMC=y