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/MCR3000_defconfig b/configs/MCR3000_defconfig
index 8484968..7aaf339 100644
--- a/configs/MCR3000_defconfig
+++ b/configs/MCR3000_defconfig
@@ -37,6 +37,7 @@
 CONFIG_SYS_OR7_PRELIM=0xFFFF810A
 CONFIG_SYS_IMMR=0xFF000000
 CONFIG_OF_BOARD_SETUP=y
+CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_BOOTDELAY=5
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="S3K> "