Allow building aarch64 images

Allow building aarch64 images

It must be noted that Docker automated build cannot create these
images as the prerequisites are unmet to cross-compile [1].

It also publishes a travis config file [2].

[1] https://www.kernel.org/doc/html/v4.11/admin-guide/binfmt-misc.html
[2] https://travis-ci.org/collivier/functest/builds/286932633

Conflicts:
    build.sh

Change-Id: I11fdef410cc40989132205a098410175f789fdef
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>
(cherry picked from commit ed777b643b05fbb71851df0b883483e0bcaa4ac1)