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/at91sam9x5ek_spiflash_defconfig b/configs/at91sam9x5ek_spiflash_defconfig
index f8d2ce3..48c2559 100644
--- a/configs/at91sam9x5ek_spiflash_defconfig
+++ b/configs/at91sam9x5ek_spiflash_defconfig
@@ -5,6 +5,7 @@
 CONFIG_DEFAULT_DEVICE_TREE="at91sam9g35ek"
 CONFIG_DEBUG_UART=y
 CONFIG_SYS_EXTRA_OPTIONS="AT91SAM9X5,SYS_USE_SPIFLASH"
+CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_BOOTDELAY=3
 # CONFIG_CONSOLE_MUX is not set
 CONFIG_SYS_CONSOLE_IS_IN_ENV=y