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/MPC8541CDS_legacy_defconfig b/configs/MPC8541CDS_legacy_defconfig
index 09a9c0f..cbf9eff 100644
--- a/configs/MPC8541CDS_legacy_defconfig
+++ b/configs/MPC8541CDS_legacy_defconfig
@@ -5,6 +5,7 @@
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_OF_STDOUT_VIA_ALIAS=y
 CONFIG_SYS_EXTRA_OPTIONS="LEGACY"
+CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_BOOTDELAY=10
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_I2C=y