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/hrcon_dh_defconfig b/configs/hrcon_dh_defconfig
index 8e911d4..48456b2 100644
--- a/configs/hrcon_dh_defconfig
+++ b/configs/hrcon_dh_defconfig
@@ -8,6 +8,7 @@
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_OF_STDOUT_VIA_ALIAS=y
 CONFIG_SYS_EXTRA_OPTIONS="HRCON_DH"
+CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_BOOTDELAY=5
 CONFIG_SYS_CONSOLE_INFO_QUIET=y
 # CONFIG_DISPLAY_BOARDINFO is not set