Move passwd and group back from /opt/etc to /etc/

Move passwd and group back from /opt/etc to /etc/

Credentials (passwd and group) files in /opt (another partiton)
instead of root introduces a depending cycle in systemd env:

1. systemd _depends_ on udev being able to notify it to mount
   /dev/mmcblk0p6 on /opt;

2. udev depends on "group" to set correctly permissions to /dev/
   nodes based on its rules.

3. credentials (passwd and group) files are in /opt, only availabe
   when mounted;

Some other information and discussion:
1. https://tizendev.org/pipermail/system/2012-November/000100.html
2. https://tizendev.org/gerrit/#/c/10005/

So we move it back to /etc (the root partition).

Another thing needs to mention is we need to make sure
{passwd,group} files available in "%install" section instead of
doing that in "%post", since otherwise rpms installed after this
package will fail for rpm keyword "%attr". e.g.:
"%attr(4750,root,dbus)" in dbus package.

Signed-off-by: Yin Kangkai <kangkai.yin@intel.com>
Signed-off-by: Karol Lewandowski <k.lewandowsk@samsung.com>
Signed-off-by: Patrick McCarty <patrick.mccarty@linux.intel.com>
Change-Id: I29fd2d20f6099307603de74a22a8a27f508aba48