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/twister_defconfig b/configs/twister_defconfig
index ad6cd96..6d67f32 100644
--- a/configs/twister_defconfig
+++ b/configs/twister_defconfig
@@ -4,6 +4,7 @@
 CONFIG_FIT=y
 CONFIG_BOOTDELAY=10
 CONFIG_SPL=y
+# CONFIG_SPL_EXT_SUPPORT is not set
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="twister => "
 # CONFIG_CMD_IMLS is not set