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/tao3530_defconfig b/configs/tao3530_defconfig
index e1bb1e7..48b30fc 100644
--- a/configs/tao3530_defconfig
+++ b/configs/tao3530_defconfig
@@ -3,6 +3,7 @@
 CONFIG_TARGET_TAO3530=y
 CONFIG_BOOTDELAY=3
 CONFIG_SPL=y
+# CONFIG_SPL_EXT_SUPPORT is not set
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="TAO-3530 # "
 # CONFIG_CMD_IMI is not set