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/zynq_zc706_defconfig b/configs/zynq_zc706_defconfig
index 825346a..35722e9 100644
--- a/configs/zynq_zc706_defconfig
+++ b/configs/zynq_zc706_defconfig
@@ -6,6 +6,7 @@
 CONFIG_FIT=y
 CONFIG_FIT_SIGNATURE=y
 CONFIG_FIT_VERBOSE=y
+CONFIG_ENV_IS_IN_SPI_FLASH=y
 # CONFIG_DISPLAY_CPUINFO is not set
 CONFIG_SPL=y
 CONFIG_SPL_OS_BOOT=y