After flashing the board, we are booting with first-install.target and
executing first-install.service. Then we are switching from first-install
to multi-user target without rebooting.
This causes issues around some services (as systemd-timedated) that are not
started correctly.

This patch removes the first-install.target and makes the board boot on
multi-user.target. It also moves the first-install.service to
default.target (multi-user.target).
Code refactoring has also been done with first-install and ota-update
services. There is only one single service now called post-install.

Tracked-On: EDISON-2430
Signed-off-by: Rodriguez Fabien <fabienx.rodriguez@intel.com>