build: add CircleCI

build: add CircleCI

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>