Convert CONFIG_SPL_EXT_SUPPORT to Kconfig

Move this option to Kconfig and tidy up existing uses.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/novena_defconfig b/configs/novena_defconfig
index d13dcb2..af84529 100644
--- a/configs/novena_defconfig
+++ b/configs/novena_defconfig
@@ -1,6 +1,7 @@
 CONFIG_ARM=y
 CONFIG_ARCH_MX6=y
 CONFIG_TARGET_KOSAGI_NOVENA=y
+CONFIG_SPL_EXT_SUPPORT=y
 CONFIG_FIT=y
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=arch/arm/imx-common/spl_sd.cfg,MX6Q"
 CONFIG_VERSION_VARIABLE=y