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/kmcoge5ne_defconfig b/configs/kmcoge5ne_defconfig
index 22cf7df..25185d6 100644
--- a/configs/kmcoge5ne_defconfig
+++ b/configs/kmcoge5ne_defconfig
@@ -4,6 +4,7 @@
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_OF_STDOUT_VIA_ALIAS=y
 CONFIG_SYS_EXTRA_OPTIONS="KMCOGE5NE"
+CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_VERSION_VARIABLE=y
 CONFIG_HUSH_PARSER=y
 CONFIG_AUTOBOOT_KEYED=y