Bug 1085631 part1: Replace b2g-config by target.

In practice, there is no much difference between b2g-config and target
mozharness options, and we are fine specifying both with the same value.

We replace b2g-config by target in our mach command interface because
it seems easier to understand and will make the life for phone builds
easier.
Bug 1085631 part2: Allow the use of a custom build script.

If the image folder contains an inner build.sh script, runs it to build
the image. The tag value is passed through a "-t" argument. All
arguments passed after the folder parameter are forwarded to the inner
build script.
Bug 1085631 part3: Add phone builder docker image.

As this image handles files containing partners blobs, it must be
private in quay.io, and the builds cannot be made public either.

You must use "-i" and "-k" build parameters to handle s3 credentials.
Bug 1085631 part4: Add the phone build task.