ntp: fix path to drift file

The default path of ntp drift file is /etc/ntp.drift, ntp daemon
maybe fails to create this file since the user ntp is not always
permitted to write /etc.

Refer to other distributions such as RedHat, Debian, just moving
the file to /var/lib/ntp which the home dir of user ntp.

Signed-off-by: Wenzong Fan <wenzong.fan@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>