bwm-ng: add a hash file

It will clarify errors like this one:
  http://autobuild.buildroot.net/results/f22/f22888d87659dda8ff75f25490eab0a68f02bc9b/



Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>