weston-init: Use weston-launch when starting weston as the first windowing system

When  weston is started as the first windowing system (i.e. not under X
nor under another Wayland server), it should be done with  the  command
weston-launch to set up proper privileged access to devices.

Signed-off-by: Tom Hochstein <tom.hochstein@nxp.com>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Ross Burton <ross.burton@intel.com>