screen-share: Add option to start screen sharing on weston startup

Currently the screen sharing has to be manually started after weston has
started by pressing Ctrl+Alt+S on a keyboard. Add new bool config option
to the screen-share section which permits starting the screen sharing of
all outputs on weston start up. This is useful e.g. for doing screen
mirroring between two devices over network.

Signed-off-by: Marek Vasut <marex@denx.de>