Use weston-init to startup weston

Use weston-init to startup weston

* weston.service should not be a part of weston recipes
  * remove weston.service from AGL weston recipes
* yocto upstream uses the recipe weston-init to startup weston
 * customize weston-start with sed
  - remove openvt
  - add option to weston-launch
 * customize weston.service with sed
   - add option for weston
* unified the weston startup beteween AGL BSPs
 * remove fix on the AGL BSP

Bug-AGL: SPEC-452
Bug-AGL: SPEC-310

Change-Id: I12722826abdc004214d289961e900c36528ec6e3
Signed-off-by: Ronan Le Martret <ronan.lemartret@iot.bzh>