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/cl-som-am57x_defconfig b/configs/cl-som-am57x_defconfig
index bf753bc..0d1f614 100644
--- a/configs/cl-som-am57x_defconfig
+++ b/configs/cl-som-am57x_defconfig
@@ -3,6 +3,7 @@
 CONFIG_OMAP54XX=y
 CONFIG_TARGET_CL_SOM_AM57X=y
 # CONFIG_SPL_NAND_SUPPORT is not set
+CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_VERSION_VARIABLE=y
 CONFIG_SPL=y
 # CONFIG_SPL_EXT_SUPPORT is not set