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/UCP1020_SPIFLASH_defconfig b/configs/UCP1020_SPIFLASH_defconfig
index 29bf82a..e03ba47 100644
--- a/configs/UCP1020_SPIFLASH_defconfig
+++ b/configs/UCP1020_SPIFLASH_defconfig
@@ -6,6 +6,7 @@
 CONFIG_FIT_VERBOSE=y
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_OF_STDOUT_VIA_ALIAS=y
+CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_HUSH_PARSER=y
 CONFIG_AUTOBOOT_KEYED=y