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-88f3720_defconfig b/configs/mvebu_db-88f3720_defconfig
index f2b2e5d..f0b196a 100644
--- a/configs/mvebu_db-88f3720_defconfig
+++ b/configs/mvebu_db-88f3720_defconfig
@@ -6,6 +6,7 @@
 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