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/at91sam9g20ek_dataflash_cs0_defconfig b/configs/at91sam9g20ek_dataflash_cs0_defconfig
index 3df66b6..c7192a7 100644
--- a/configs/at91sam9g20ek_dataflash_cs0_defconfig
+++ b/configs/at91sam9g20ek_dataflash_cs0_defconfig
@@ -5,6 +5,7 @@
 CONFIG_DEFAULT_DEVICE_TREE="at91sam9g20ek"
 CONFIG_DEBUG_UART=y
 CONFIG_SYS_EXTRA_OPTIONS="AT91SAM9G20,SYS_USE_DATAFLASH_CS0"
+CONFIG_ENV_IS_IN_DATAFLASH=y
 CONFIG_BOOTDELAY=3
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_SYS_PROMPT="U-Boot> "