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/MPC8315ERDB_defconfig b/configs/MPC8315ERDB_defconfig
index 52ce29a..78e9b33 100644
--- a/configs/MPC8315ERDB_defconfig
+++ b/configs/MPC8315ERDB_defconfig
@@ -3,6 +3,7 @@
 CONFIG_TARGET_MPC8315ERDB=y
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_OF_STDOUT_VIA_ALIAS=y
+CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_BOOTDELAY=6
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_I2C=y