.travis.yml: enable OSX builds

Travis has support for OSX builds. Making the setup work cleanly
involves a little hacking about with the .travis.yml file but rather
than make it too messy I've pushed all the "brew" install stuff into a
support script called ./scripts/macosx-brew.sh.

Currently only the default ./configure ${CONFIG} is built as I'm not
sure what extra coverage would come from the other build stanzas.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Acked-by: Peter Maydell <peter.maydell@linaro.org>
