Use variable for service install location

After enabling 'usrmerge', the install location for systemd service
files changed. This uses a variable to get the installation location
instead of hardcoding it.

Tested:
Confirmed that with 'usrmerge' enabled, the service file is correctly
installed.

Change-Id: I80eeca283ae4b7f58461b220ab7879ab3a150943
Signed-off-by: Jason M. Bills <jason.m.bills@intel.com>
1 file changed