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/mx35pdk_defconfig b/configs/mx35pdk_defconfig
index 324a4db..11bc00c 100644
--- a/configs/mx35pdk_defconfig
+++ b/configs/mx35pdk_defconfig
@@ -1,5 +1,6 @@
 CONFIG_ARM=y
 CONFIG_TARGET_MX35PDK=y
+CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_BOOTDELAY=1
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_BOARD_EARLY_INIT_F=y