Add cross-compilation testing to .travis.yml

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Separate nsinit main from implementation. This is done inorder to package nsinit as part of docker binary.

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
Merge pull request #61 from vishh/split_nsinit

Separate nsinit main from implementation
Merge pull request #60 from tianon/cross-travis

Add cross-compilation testing to .travis.yml
Factor out setting of allowed devices so that we can update allowed devices

Co-Authored-By: Ian Main <imain@redhat.com>
Docker-DCO-1.1-Signed-off-by: Chris Alfonso <calfonso@redhat.com> (github: calfonso)