Bug 1085620 part2: Support for emulator builds in the builder image.

The setup of builder image moved to system-setup.sh script because
needs to do more complex work, like downloading repo and installing
mozilla buildtools package.

The build-emulator.sh script is reponsible for building emulators.
Besides the REPOSITORY and REVISION environment variables, it requires
TARGET and B2G_CONFIG too.
Bug 1085620 part3: Add emulator build task.

The "mach ci-build" has a new optional argument called b2g-config. This
allows the user to pass the b2g config directory for the target. This
argument is necessary for emulator and phone builds.