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/colibri_pxa270_defconfig b/configs/colibri_pxa270_defconfig
index ddf4bf6..040408b 100644
--- a/configs/colibri_pxa270_defconfig
+++ b/configs/colibri_pxa270_defconfig
@@ -1,5 +1,6 @@
 CONFIG_ARM=y
 CONFIG_TARGET_COLIBRI_PXA270=y
+CONFIG_ENV_IS_IN_FLASH=y
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_SYS_PROMPT="$ "
 # CONFIG_CMD_ELF is not set