Convert CONFIG_ENV_IS_IN_SPI_FLASH to Kconfig

This converts the following to Kconfig:
   CONFIG_ENV_IS_IN_SPI_FLASH

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/ot1200_spl_defconfig b/configs/ot1200_spl_defconfig
index 531fb33..50be468 100644
--- a/configs/ot1200_spl_defconfig
+++ b/configs/ot1200_spl_defconfig
@@ -9,6 +9,7 @@
 CONFIG_SPL_SPI_SUPPORT=y
 CONFIG_SPL_WATCHDOG_SUPPORT=y
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=arch/arm/mach-imx/spl_sd.cfg,MX6Q"
+CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_BOOTDELAY=3
 CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_SPL=y