Even when configured to only build PR, CircleCI always build the default branch
`only: /pull.*/` seems like a good idea, but it fails when:
- you are testing on the same repo, because github doesn't create a pull branch
- when your repo is configured, because it see the push on your repo,
prevent building this commit because it doesn't match the regexp, then when you
do the PR CircleCI doesn't work
In the end what we really want is `ignore: master`

Signed-off-by: Etienne Champetier <champetier.etienne@gmail.com>

Signed-off-by: Etienne Champetier <champetier.etienne@gmail.com>

Signed-off-by: Etienne Champetier <champetier.etienne@gmail.com>

The big advantages are:
- everyone can download the build logs and the ipks
- we use our own docker image
- people with commit access can ssh into the build env

The disadvantages:
- need to push new commits to restart the build

I haven't reimplemented the commit message checks as this should be replaced
with a separate script doing only that so we can require it
(https://help.github.com/articles/enabling-required-status-checks/)

Signed-off-by: Etienne Champetier <champetier.etienne@gmail.com>