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/birdland_bav335a_defconfig b/configs/birdland_bav335a_defconfig
index 79c2ac1..43b46b9 100644
--- a/configs/birdland_bav335a_defconfig
+++ b/configs/birdland_bav335a_defconfig
@@ -1,5 +1,6 @@
 CONFIG_ARM=y
 CONFIG_TARGET_BAV335X=y
+CONFIG_SPL_EXT_SUPPORT=y
 CONFIG_BAV_VERSION=1
 CONFIG_FIT=y
 CONFIG_SYS_EXTRA_OPTIONS="SERIAL1,CONS_INDEX=1"