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/mcx_defconfig b/configs/mcx_defconfig
index c6ac8d4..8378832 100644
--- a/configs/mcx_defconfig
+++ b/configs/mcx_defconfig
@@ -4,6 +4,7 @@
 CONFIG_FIT=y
 CONFIG_BOOTDELAY=3
 CONFIG_SPL=y
+# CONFIG_SPL_EXT_SUPPORT is not set
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="mcx # "
 # CONFIG_CMD_IMI is not set