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/cm_t43_defconfig b/configs/cm_t43_defconfig
index a2ffcb2..f52ee36 100644
--- a/configs/cm_t43_defconfig
+++ b/configs/cm_t43_defconfig
@@ -1,7 +1,8 @@
 CONFIG_ARM=y
 CONFIG_TARGET_CM_T43=y
-CONFIG_SPL=y
+CONFIG_SPL_EXT_SUPPORT=y
 CONFIG_VERSION_VARIABLE=y
+CONFIG_SPL=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="CM-T43 # "
 CONFIG_CMD_BOOTZ=y