make USER limited to 6 characters when generating BF_BUILD_NUMBER

the value of USER is dependent from the compilation environment,so
when compiling one same device project, the BUILD_FINGERPRINT may
exceed 91 characters because ${USER} is long, but with short ${USER}
the compilation can pass.

Signed-off-by: wei qiao <qiaowei224@gmail.com>
Change-Id: Ia0f7dfa9cf7d605f1f2603f70dd0e6877482eb8a