tools: check in certdata.txt from mozilla NSS

This is a file from the NSS project containing root certificate data.
It can be downloaded from:

  https://mxr.mozilla.org/mozilla/source/security/nss/lib/ckfw/builtins/certdata.txt?raw=1

Once downloaded, running `perl tools/mk-ca-bundle.pl` will update
src/node_root_certs.h.  Commit, rebuild and go.