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/db-88f6820-amc_defconfig b/configs/db-88f6820-amc_defconfig
index 3831f40..ab42498 100644
--- a/configs/db-88f6820-amc_defconfig
+++ b/configs/db-88f6820-amc_defconfig
@@ -10,6 +10,7 @@
 CONFIG_DEBUG_UART=y
 CONFIG_FIT=y
 CONFIG_FIT_VERBOSE=y
+CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_BOOTDELAY=3
 CONFIG_SYS_CONSOLE_INFO_QUIET=y
 # CONFIG_DISPLAY_BOARDINFO is not set