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/at91sam9g10ek_dataflash_cs3_defconfig b/configs/at91sam9g10ek_dataflash_cs3_defconfig
index 8da1bfe..1597d0e 100644
--- a/configs/at91sam9g10ek_dataflash_cs3_defconfig
+++ b/configs/at91sam9g10ek_dataflash_cs3_defconfig
@@ -2,6 +2,7 @@
 CONFIG_ARCH_AT91=y
 CONFIG_TARGET_AT91SAM9261EK=y
 CONFIG_SYS_EXTRA_OPTIONS="AT91SAM9G10,SYS_USE_DATAFLASH_CS3"
+CONFIG_ENV_IS_IN_DATAFLASH=y
 CONFIG_BOOTDELAY=3
 # CONFIG_CONSOLE_MUX is not set
 CONFIG_SYS_CONSOLE_IS_IN_ENV=y