webos-features: Add extra variables to select init system

* upstart in DISTRO_FEATURES isn't currently used, but will
  be in following changes
* sysvinit in DISTRO_FEATURES is needed for newer update-rc.d
  bbclass to work correctly with our hybrid images
* WEBOS_DISTRO_FEATURES_INIT_CONSIDERED is needed to prevent
  backfilling of sysvinit when oe-core is upgraded to 1.4
* both variables are useful to change default init system
  from local.conf without changing meta-webos

Open-webOS-DCO-1.0-Signed-off-by: Martin Jansa <martin.jansa@palm.com>
Change-Id: I1b82bd2a1eaf9573b521632bfba6a5a5fe33cf6e
Reviewed-on: https://g2g.palm.com/1842
Reviewed-by: Martin Jansa <Martin.Jansa@lge.com>
Tested-by: Martin Jansa <Martin.Jansa@lge.com>
Reviewed-by: Susan Montooth <susan.montooth@lge.com>