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>

package/bwm-ng/bwm-ng.hash

@@ -0,0 +1,2 @@
+# Locally calculated
+sha256  c1134358e268329d438b0996399003b0f0b966034fb4b5b138761c2f3c62ffdd  bwm-ng-0.6.tar.gz