debian: add gbp.conf file

Signed-off-by: Nicolas Dechesne <nicolas.dechesne@linaro.org>

debian/gbp.conf

+[DEFAULT]
+# Keep pristine-tar (allows to re-create 'orig').
+pristine-tar = True
+
+# release from linaro branch
+debian-branch = linaro
+ 
+[git-import-dsc]
+# import in ubuntu branch
+debian-branch = ubuntu