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/arch/arm/cpu/armv7/am33xx/Kconfig b/arch/arm/cpu/armv7/am33xx/Kconfig
index dc51e9b..efbc219 100644
--- a/arch/arm/cpu/armv7/am33xx/Kconfig
+++ b/arch/arm/cpu/armv7/am33xx/Kconfig
@@ -1,4 +1,8 @@
 if AM43XX
+
+config SPL_EXT_SUPPORT
+	default y
+
 config TARGET_AM43XX_EVM
 	bool "Support am43xx_evm"
 	select TI_I2C_BOARD_DETECT