Define kernelversion if ext toolchain is used

toolchain/kernel-headers/kernel-headers.mk

@@ -6,8 +6,14 @@
 # by a 'make clean' so don't put anything sacred in here...
 #
 #############################################################
+ifeq ($(BR2_TOOLCHAIN_EXTERNAL),y)
+DEFAULT_KERNEL_HEADERS:=$(strip $(subst ",, $(BR2_LINUX26_VERSION)))
+#"))
+else
 DEFAULT_KERNEL_HEADERS:=$(strip $(subst ",, $(BR2_DEFAULT_KERNEL_HEADERS)))
 #"))
+endif
+
 LINUX_HEADERS_SITE:=127.0.0.1
 LINUX_HEADERS_SOURCE:=unspecified-kernel-headers
 LINUX_HEADERS_UNPACK_DIR:=$(TOOL_BUILD_DIR)/linux-libc-headers-null