useradd_base.bbclass: replace retry logic with flock

useradd_base.bbclass: replace retry logic with flock

When perform useradd during populate sysroot, it locks files passwd.lock
and group.lock at same time. And then it meets a dead lock issue
randomly.

Use flock to reslove it by using an universal lock directory for all the
user and group related operations.

[YOCTO #9022]

Signed-off-by: Kai Kang <kai.kang@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>