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/P5040DS_defconfig b/configs/P5040DS_defconfig
index b56c86e..c9d9238 100644
--- a/configs/P5040DS_defconfig
+++ b/configs/P5040DS_defconfig
@@ -5,6 +5,7 @@
 CONFIG_FIT_VERBOSE=y
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_OF_STDOUT_VIA_ALIAS=y
+CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_BOOTDELAY=10
 CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_HUSH_PARSER=y