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/mvebu_db_armada8k_defconfig b/configs/mvebu_db_armada8k_defconfig
index 7f36eda..7f4ee05 100644
--- a/configs/mvebu_db_armada8k_defconfig
+++ b/configs/mvebu_db_armada8k_defconfig
@@ -7,6 +7,7 @@
 CONFIG_DEBUG_UART=y
 CONFIG_AHCI=y
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
+CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_SYS_CONSOLE_INFO_QUIET=y
 # CONFIG_DISPLAY_CPUINFO is not set
 # CONFIG_DISPLAY_BOARDINFO is not set