Add scripts to download, verify and use GNU Arm Embedded 5 2016q2.
2016q2 (SVN revision 237715) generates identical output to Debian
Stretch used by Docker build (SVN revision 241155).
so that emulator build uses pipenv to ensure stable environment for python-trezor
but fetching python-trezor and friends doesn't needlessly slow down non-emulator builds
that don't need it