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/nitrogen6dl2g_defconfig b/configs/nitrogen6dl2g_defconfig
index 5744ede..9b8914b 100644
--- a/configs/nitrogen6dl2g_defconfig
+++ b/configs/nitrogen6dl2g_defconfig
@@ -4,6 +4,7 @@
 CONFIG_VIDEO=y
 CONFIG_CMD_HDMIDETECT=y
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=board/boundary/nitrogen6x/nitrogen6dl2g.cfg,MX6DL,DDR_MB=2048"
+CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_BOOTDELAY=3
 CONFIG_SYS_CONSOLE_OVERWRITE_ROUTINE=y
 # CONFIG_SYS_STDIO_DEREGISTER is not set