Convert CONFIG_ENV_IS_IN_DATAFLASH to Kconfig

This converts the following to Kconfig:
   CONFIG_ENV_IS_IN_DATAFLASH

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/meesc_dataflash_defconfig b/configs/meesc_dataflash_defconfig
index de8a8cb..146838e 100644
--- a/configs/meesc_dataflash_defconfig
+++ b/configs/meesc_dataflash_defconfig
@@ -3,6 +3,7 @@
 CONFIG_TARGET_MEESC=y
 CONFIG_FIT=y
 CONFIG_SYS_EXTRA_OPTIONS="AT91SAM9263,SYS_USE_DATAFLASH"
+CONFIG_ENV_IS_IN_DATAFLASH=y
 CONFIG_BOOTDELAY=3
 CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_HUSH_PARSER=y