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/MPC8308RDB_defconfig b/configs/MPC8308RDB_defconfig
index c0f994a..10ff9cf 100644
--- a/configs/MPC8308RDB_defconfig
+++ b/configs/MPC8308RDB_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=5
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_MMC=y