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_defconfig b/configs/MPC8541CDS_defconfig
index 8268388..2d4647a 100644
--- a/configs/MPC8541CDS_defconfig
+++ b/configs/MPC8541CDS_defconfig
@@ -4,6 +4,7 @@
 CONFIG_TARGET_MPC8541CDS=y
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_OF_STDOUT_VIA_ALIAS=y
+CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_BOOTDELAY=10
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_I2C=y