weston-init: Pass --continue-without-input when launching weston

This ensures that we do not need keyboard/mouse or other input system
Weston's default behavior is to not open if input devices are not
found, but we may not always have input devices connected for systems
running weston in field

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>