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/ls1043ardb_defconfig b/configs/ls1043ardb_defconfig
index 894110b..4050737 100644
--- a/configs/ls1043ardb_defconfig
+++ b/configs/ls1043ardb_defconfig
@@ -4,6 +4,7 @@
 CONFIG_DEFAULT_DEVICE_TREE="fsl-ls1043a-rdb"
 CONFIG_FIT_VERBOSE=y
 CONFIG_OF_BOARD_SETUP=y
+CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_BOOTDELAY=10
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_GPT=y