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/ethernut5_defconfig b/configs/ethernut5_defconfig
index 197052d..2bf06ad 100644
--- a/configs/ethernut5_defconfig
+++ b/configs/ethernut5_defconfig
@@ -2,6 +2,7 @@
 CONFIG_ARCH_AT91=y
 CONFIG_TARGET_ETHERNUT5=y
 CONFIG_SYS_EXTRA_OPTIONS="AT91SAM9XE"
+CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_BOOTDELAY=3
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_HUSH_PARSER=y