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/tqma6dl_mba6_spi_defconfig b/configs/tqma6dl_mba6_spi_defconfig
index dee295e..35b26a3 100644
--- a/configs/tqma6dl_mba6_spi_defconfig
+++ b/configs/tqma6dl_mba6_spi_defconfig
@@ -6,6 +6,7 @@
 CONFIG_FIT=y
 CONFIG_FIT_VERBOSE=y
 CONFIG_OF_BOARD_SETUP=y
+CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_BOOTDELAY=3
 CONFIG_DEFAULT_FDT_FILE="imx6dl-mba6x.dtb"
 CONFIG_BOARD_EARLY_INIT_F=y