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/usb_a9263_dataflash_defconfig b/configs/usb_a9263_dataflash_defconfig
index 7a8005b..edadef3 100644
--- a/configs/usb_a9263_dataflash_defconfig
+++ b/configs/usb_a9263_dataflash_defconfig
@@ -2,6 +2,7 @@
 CONFIG_ARCH_AT91=y
 CONFIG_TARGET_USB_A9263=y
 CONFIG_SYS_EXTRA_OPTIONS="AT91SAM9263,SYS_USE_DATAFLASH"
+CONFIG_ENV_IS_IN_DATAFLASH=y
 CONFIG_BOOTDELAY=3
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_HUSH_PARSER=y