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/vexpress_ca9x4_defconfig b/configs/vexpress_ca9x4_defconfig
index 7b845c6..851f034 100644
--- a/configs/vexpress_ca9x4_defconfig
+++ b/configs/vexpress_ca9x4_defconfig
@@ -1,6 +1,7 @@
 CONFIG_ARM=y
 CONFIG_TARGET_VEXPRESS_CA9X4=y
 CONFIG_DISTRO_DEFAULTS=y
+CONFIG_ENV_IS_IN_FLASH=y
 # CONFIG_DISPLAY_CPUINFO is not set
 # CONFIG_DISPLAY_BOARDINFO is not set
 # CONFIG_CMD_CONSOLE is not set