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_shc_netboot_defconfig b/configs/am335x_shc_netboot_defconfig
index 990bd4d..ca14727 100644
--- a/configs/am335x_shc_netboot_defconfig
+++ b/configs/am335x_shc_netboot_defconfig
@@ -1,5 +1,6 @@
 CONFIG_ARM=y
 CONFIG_TARGET_AM335X_SHC=y
+CONFIG_SPL_EXT_SUPPORT=y
 CONFIG_SPL_ENV_SUPPORT=y
 CONFIG_SHC_NETBOOT=y
 CONFIG_SERIES=y