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/sama5d4ek_spiflash_defconfig b/configs/sama5d4ek_spiflash_defconfig
index 98813bc..7c6c8f2 100644
--- a/configs/sama5d4ek_spiflash_defconfig
+++ b/configs/sama5d4ek_spiflash_defconfig
@@ -13,6 +13,7 @@
 CONFIG_DEBUG_UART=y
 CONFIG_FIT=y
 CONFIG_SYS_EXTRA_OPTIONS="SAMA5D4,SYS_USE_SERIALFLASH"
+CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_BOOTDELAY=3
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_SPL=y