kernel-defaults: Delete external source tree user_headers

When we use an external kernel tree which may not have been fully
cleaned, there may be user_headers left which do not match the target
architecture, leading to build failures for packages that do an explicit
inclusion of user_headers (such as iproute2 or iptables). Make sure we
delete them while preparing the directory.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>

include/kernel-defaults.mk

@@ -46,6 +46,7 @@ else
 		rmdir $(LINUX_DIR); \
 	fi
 	ln -s $(CONFIG_EXTERNAL_KERNEL_TREE) $(LINUX_DIR)
+	$(_SINGLE) [ -d $(LINUX_DIR)/user_headers ] && rm -rf $(LINUX_DIR)/user_headers
   endef
 endif