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/vct_platinum_small_defconfig b/configs/vct_platinum_small_defconfig
index 2b49d4c..72f53e0 100644
--- a/configs/vct_platinum_small_defconfig
+++ b/configs/vct_platinum_small_defconfig
@@ -2,6 +2,7 @@
 CONFIG_TARGET_VCT=y
 CONFIG_VCT_PLATINUM=y
 CONFIG_VCT_SMALL_IMAGE=y
+CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_BOOTDELAY=5
 CONFIG_SYS_CONSOLE_INFO_QUIET=y
 CONFIG_SYS_PROMPT="$ "