.travis.yml: separate tools and docs into another entry

.travis.yml: separate tools and docs into another entry

Re-building the tools and documents by default is a little wasteful as
they are not really affected by the main build options. Split tools
and documents into their own task with a minimal softmmu and
linux-user target list just to check they don't interact badly.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>