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/pcm058_defconfig b/configs/pcm058_defconfig
index d18faf7..dc3fed7 100644
--- a/configs/pcm058_defconfig
+++ b/configs/pcm058_defconfig
@@ -13,6 +13,7 @@
 CONFIG_CMD_HDMIDETECT=y
 CONFIG_FIT=y
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=arch/arm/mach-imx/spl_sd.cfg,MX6Q"
+CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_BOOTDELAY=3
 CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_SPL=y