webos-image: add missing pieces to support to support selecting an alternative to dropbear

Open-webOS-DCO-1.0-Signed-off-by: Simon Busch <morphis@gravedo.de>
webos-image: pull dropbear through ssh-server-dropbear image feature

* it was in task-webos-core RDEPENDS before

Open-webOS-DCO-1.0-Signed-off-by: Simon Busch <morphis@gravedo.de>
task-webos-extended: remove dropbear

* sshd (openssh or dropbear) should be decided by IMAGE_FEATURE

Open-webOS-DCO-1.0-Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>