Add variables for path of systemd's unit files

To use systemd user mode from recipes, it convenient that
some variables are available, which point proper paths for unit files.
For that, backport 2 variables which has been added latest poky.

(From Poky rev:319a69f1d757a577b50a2ca9c8fbf845eba8e274)

When upgrading poky in future, these variables should be removed
from poky-agl.conf.

Change-Id: I8ba7594b488ecd6421caf7dde484eb6b091ff0de
Signed-off-by: Tadao Tanikawa <tanikawa.tadao@jp.panasonic.com>