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/board/ti/common/Kconfig b/board/ti/common/Kconfig
index 77c92a1..5173e6b 100644
--- a/board/ti/common/Kconfig
+++ b/board/ti/common/Kconfig
@@ -6,3 +6,6 @@
 	help
 	   Support for detection board information on Texas Instrument's
 	   Evaluation Boards which have I2C based EEPROM detection
+
+config SPL_EXT_SUPPORT
+	default y