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_defconfig b/configs/vct_platinum_defconfig
index f8b9d7e..dbb4bcf 100644
--- a/configs/vct_platinum_defconfig
+++ b/configs/vct_platinum_defconfig
@@ -1,6 +1,7 @@
 CONFIG_MIPS=y
 CONFIG_TARGET_VCT=y
 CONFIG_VCT_PLATINUM=y
+CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_BOOTDELAY=5
 CONFIG_SYS_CONSOLE_INFO_QUIET=y
 CONFIG_SYS_PROMPT="$ "