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/ap325rxa_defconfig b/configs/ap325rxa_defconfig
index ebefd12..ebefe3c 100644
--- a/configs/ap325rxa_defconfig
+++ b/configs/ap325rxa_defconfig
@@ -1,5 +1,6 @@
 CONFIG_SH=y
 CONFIG_TARGET_AP325RXA=y
+CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_BOOTDELAY=3
 CONFIG_VERSION_VARIABLE=y
 # CONFIG_CMD_BDI is not set