make USER limited to 6 characters when generating BUILD_NUMBER

BUILD_NUMBER is use to generate ro.build.description and
ro.build.display.id, if USER is long, these properties will
exceed 91 characters easily.

Change-Id: I7f99ef351f80210172a873104edd980b36c3f8f1
Signed-off-by: weiqiao <qiaowei224@gmail.com>