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/mx6slevk_spinor_defconfig b/configs/mx6slevk_spinor_defconfig
index 67d62f8..9260677 100644
--- a/configs/mx6slevk_spinor_defconfig
+++ b/configs/mx6slevk_spinor_defconfig
@@ -5,6 +5,7 @@
 CONFIG_DEFAULT_DEVICE_TREE="imx6sl-evk"
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=board/freescale/mx6slevk/imximage.cfg,MX6SL"
 CONFIG_SPI_BOOT=y
+CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_BOOTDELAY=3
 CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_HUSH_PARSER=y