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/sama5d36ek_cmp_spiflash_defconfig b/configs/sama5d36ek_cmp_spiflash_defconfig
index 8db4f52..53bab32 100644
--- a/configs/sama5d36ek_cmp_spiflash_defconfig
+++ b/configs/sama5d36ek_cmp_spiflash_defconfig
@@ -6,6 +6,7 @@
 CONFIG_DEBUG_UART=y
 CONFIG_FIT=y
 CONFIG_SYS_EXTRA_OPTIONS="SAMA5D3,SYS_USE_SERIALFLASH"
+CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_BOOTDELAY=3
 # CONFIG_CONSOLE_MUX is not set
 CONFIG_SYS_CONSOLE_IS_IN_ENV=y