buildroot: filter out dtbs KERNELNAME target (#13785)

Do not attempt to copy arch/$(LINUX_KARCH)/boot/dtbs because these are
not real files, fixes #13785.

Signed-off-by: Florian Fainelli <florian@openwrt.org>

SVN-Revision: 37107

include/kernel-defaults.mk

@@ -128,7 +128,7 @@ define Kernel/CopyImage
 	$(KERNEL_CROSS)objcopy $(OBJCOPY_STRIP) -S $(LINUX_DIR)/vmlinux $(KERNEL_BUILD_DIR)/vmlinux$(1).elf
 ifneq ($(subst ",,$(KERNELNAME)),)
 	#")
-	$(foreach k,$(subst ",,$(KERNELNAME)),$(CP) $(LINUX_DIR)/arch/$(LINUX_KARCH)/boot/$(IMAGES_DIR)/$(k) $(KERNEL_BUILD_DIR)/$(k)$(1);)
+	$(foreach k,$(filter-out dtbs,$(subst ",,$(KERNELNAME))),$(CP) $(LINUX_DIR)/arch/$(LINUX_KARCH)/boot/$(IMAGES_DIR)/$(k) $(KERNEL_BUILD_DIR)/$(k)$(1);)
 	#")
 endif
 endef