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
diff --git a/core/Makefile b/core/Makefile
index ad577a9..67ad228 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -122,7 +122,7 @@
   ifneq ($(filter eng.%,$(BUILD_NUMBER)),)
     # Trim down BUILD_FINGERPRINT: the default BUILD_NUMBER makes it easily exceed
     # the Android system property length limit (PROPERTY_VALUE_MAX=92).
-    BF_BUILD_NUMBER := $(USER)$(shell $(DATE) +%m%d%H%M)
+    BF_BUILD_NUMBER := $(shell echo $${USER:0:6})$(shell $(DATE) +%m%d%H%M)
   else
     BF_BUILD_NUMBER := $(BUILD_NUMBER)
   endif