build: add a default build tar in depends

This is so we can override it later for BSDs.

depends/Makefile

@@ -182,7 +182,7 @@ $(host_prefix)/.stamp_$(final_build_id): $(native_packages) $(packages)
 	mkdir -p $(@D)
 	echo copying packages: $^
 	echo to: $(@D)
-	cd $(@D); $(foreach package,$^, tar xf $($(package)_cached); )
+	cd $(@D); $(foreach package,$^, $(build_TAR) xf $($(package)_cached); )
 	touch $@
 
 # $PATH is not preserved between ./configure and make by convention. Its