webos-image: pull dropbear through ssh-server-dropbear

* it was in task-webos-extended RDEPENDS and before
  that in task-webos-core RDEPENDS, but sshd in image
  (openssh or dropbear) should be decided by IMAGE_FEATURES

Open-webOS-DCO-1.0-Signed-off-by: Martin Jansa <martin.jansa@gmail.com>
Change-Id: I8d3cba130caafa9d0e5dce225206fb2442e8f13e
Reviewed-on: http://g2g.palm.com/1095
Reviewed-by: Herb Kuta <herb.kuta@palm.com>
Tested-by: Herb Kuta <herb.kuta@palm.com>