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/am335x_igep0033_defconfig b/configs/am335x_igep0033_defconfig
index efa3a9c..f12f1dc 100644
--- a/configs/am335x_igep0033_defconfig
+++ b/configs/am335x_igep0033_defconfig
@@ -1,5 +1,6 @@
 CONFIG_ARM=y
 CONFIG_TARGET_AM335X_IGEP0033=y
+CONFIG_SPL_EXT_SUPPORT=y
 CONFIG_SPL_STACK_R_ADDR=0x82000000
 CONFIG_SPL=y
 CONFIG_SPL_STACK_R=y