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/ap143_defconfig b/configs/ap143_defconfig
index 3d856e9..ca77e79 100644
--- a/configs/ap143_defconfig
+++ b/configs/ap143_defconfig
@@ -4,6 +4,7 @@
 CONFIG_TARGET_AP143=y
 CONFIG_DEFAULT_DEVICE_TREE="ap143"
 CONFIG_DEBUG_UART=y
+CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_BOOTDELAY=3
 CONFIG_DISPLAY_CPUINFO=y
 CONFIG_BOARD_EARLY_INIT_F=y