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/10m50_defconfig b/configs/10m50_defconfig
index ed9a867..465edc5 100644
--- a/configs/10m50_defconfig
+++ b/configs/10m50_defconfig
@@ -2,6 +2,7 @@
 CONFIG_SYS_CONFIG_NAME="10m50_devboard"
 CONFIG_DEFAULT_DEVICE_TREE="10m50_devboard"
 CONFIG_FIT=y
+CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_SYS_CONSOLE_INFO_QUIET=y
 CONFIG_VERSION_VARIABLE=y
 CONFIG_HUSH_PARSER=y