external: Delete source dirs before re-extracting tarballs

If the timestamp on the dist/ tarball gets changed (e.g. via git rebase),
it can cause the $(IPTABLES_SRC)/configure or $(BUSYBOX_BUILD)/.unpacked
targets to get re-invoked.  These should start with a clean source
directory to avoid patch conflicts on "newly added" files like
libxt_quota2.c.
external: iptables: Add workaround for broken HTC One SV libc
res: Update iptables binaries
travis: First attempt at getting debug builds up