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/cgtqmx6eval_defconfig b/configs/cgtqmx6eval_defconfig
index 85ac7ac..e20e5f8 100644
--- a/configs/cgtqmx6eval_defconfig
+++ b/configs/cgtqmx6eval_defconfig
@@ -12,6 +12,7 @@
 CONFIG_SPL_WATCHDOG_SUPPORT=y
 CONFIG_VIDEO=y
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=arch/arm/mach-imx/spl_sd.cfg,MX6QDL"
+CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_BOOTDELAY=3
 # CONFIG_CONSOLE_MUX is not set
 CONFIG_SYS_CONSOLE_IS_IN_ENV=y