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/flea3_defconfig b/configs/flea3_defconfig
index 0b76e11..75a97e3 100644
--- a/configs/flea3_defconfig
+++ b/configs/flea3_defconfig
@@ -2,6 +2,7 @@
 CONFIG_TARGET_FLEA3=y
 CONFIG_FIT=y
 CONFIG_OF_BOARD_SETUP=y
+CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_BOOTDELAY=3
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_BOARD_EARLY_INIT_F=y