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/spear600_defconfig b/configs/spear600_defconfig
index 7b8cd06..3c5c0e6 100644
--- a/configs/spear600_defconfig
+++ b/configs/spear600_defconfig
@@ -2,6 +2,7 @@
 CONFIG_TARGET_SPEAR600=y
 CONFIG_IDENT_STRING="-SPEAr"
 CONFIG_SYS_EXTRA_OPTIONS="SPEAR600"
+CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_BOOTDELAY=1
 CONFIG_SYS_CONSOLE_INFO_QUIET=y
 # CONFIG_DISPLAY_BOARDINFO is not set