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/ms7722se_defconfig b/configs/ms7722se_defconfig
index 92f3158..633a087 100644
--- a/configs/ms7722se_defconfig
+++ b/configs/ms7722se_defconfig
@@ -1,5 +1,6 @@
 CONFIG_SH=y
 CONFIG_TARGET_MS7722SE=y
+CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_BOOTDELAY=3
 CONFIG_VERSION_VARIABLE=y
 # CONFIG_CMD_BDI is not set