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/x600_defconfig b/configs/x600_defconfig
index 2e5d5f1..d766614 100644
--- a/configs/x600_defconfig
+++ b/configs/x600_defconfig
@@ -7,6 +7,7 @@
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
 CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_IDENT_STRING="-SPEAr"
+CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_BOOTDELAY=3
 CONFIG_SYS_CONSOLE_INFO_QUIET=y
 # CONFIG_DISPLAY_BOARDINFO is not set