Make b2g harness able to check multiple devices from the same computer:
 - Isolate device-specific configuration in $B2G_DIR/perso
 - Add support for flashing / testing multiple devices at once
   by reseting the abd serial to the fastboot serial.
 - Clone a local copy of the gaia-ui-tests.
 - Move Python install to the device-specific directory.
 - Expect the directory of the B2G repo as argument, and make the
   awfy standalone driver compatible with it.

Various Clean-up & factorisation:
 - Setup the implicit symbolc link for the manifest file.
 - Add creation of python site-packages directories.