weston-init: Redefine weston service and add socket activation option

Currently, weston when autolauched with systemd ends up with seat0
related errors because its launched before seat0 is assigned.

Fixes
[05:16:09.357] logind: failed to get session seat
[05:16:09.358] logind: cannot setup systemd-logind helper (-61), using legacy fallback

The above error results in ptest failures in parselogs, even though
weston has started ok using fallback methods, these errors are still seen in logs

Also fixes weston ptests
RESULTS - weston.WestonTest.test_weston_can_initialize_new_wayland_compositor: PASSED (8.58s)

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Cc: Marek Vasut <marex@denx.de>
Cc: Henning Heinold <henning@itconsulting-heinold.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>