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/mx6sxsabresd_spl_defconfig b/configs/mx6sxsabresd_spl_defconfig
index 1815573..a7254d0 100644
--- a/configs/mx6sxsabresd_spl_defconfig
+++ b/configs/mx6sxsabresd_spl_defconfig
@@ -1,6 +1,7 @@
 CONFIG_ARM=y
 CONFIG_ARCH_MX6=y
 CONFIG_TARGET_MX6SXSABRESD=y
+CONFIG_SPL_EXT_SUPPORT=y
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=arch/arm/imx-common/spl_sd.cfg"
 CONFIG_BOOTDELAY=3
 CONFIG_SPL=y