weston: Make keyboard optional

For optimisation purpose there is no need to launch
extra weston-keyboard on systems where input is handled differently
(eg: Using custom UI)

This can be disabled in blueprints conf if desired

Relate-to: https://forum.ostc-eu.org/t/226#bootchart
Forwarded: https://booting.oniroproject.org/distro/oniro/-/merge_requests/442


Signed-off-by: Philippe Coval <philippe.coval@huawei.com>