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/ids8313_defconfig b/configs/ids8313_defconfig
index 2a8c239..85c09f8 100644
--- a/configs/ids8313_defconfig
+++ b/configs/ids8313_defconfig
@@ -6,6 +6,7 @@
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_OF_STDOUT_VIA_ALIAS=y
 CONFIG_SYS_EXTRA_OPTIONS="SYS_TEXT_BASE=0xFFF00000"
+CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_BOOTDELAY=1
 CONFIG_VERSION_VARIABLE=y
 # CONFIG_DISPLAY_BOARDINFO is not set