Update:
Added:
[core]
shell=kiosk-shell.so
in /etc/xdg/weston/weston.ini. Now no background and not upper bar. OK what I wanted.
Regarding boot messages:
fw_setenv setupargs “fbcon=map:1 quiet”
makes HDMI boot output having some blank screen and, as before, the plymouth splash image only for few moments. Suggestions?