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/xtfpga_defconfig b/configs/xtfpga_defconfig
index 9f85d63..e52e62b 100644
--- a/configs/xtfpga_defconfig
+++ b/configs/xtfpga_defconfig
@@ -1,6 +1,7 @@
 CONFIG_XTENSA=y
 CONFIG_SYS_CPU="dc233c"
 CONFIG_XTFPGA_KC705=y
+CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_BOOTDELAY=10
 CONFIG_VERSION_VARIABLE=y
 CONFIG_SYS_PROMPT="U-Boot> "