dropbear: Persist generated keys across software updates

:Release Notes:
By default, dropbear writes its keys to /etc, which isn't persisted
across software updates. This change modifies it to write them under the
tree that is persisted across software updates.

:Detailed Notes:

:Testing Performed:
1) MiniBAT
2) This change was tested as part of the full solution for read-only
   rootfs.

:QA Notes:

:Issues Addressed:
[GF-6589] Update scripts and programs for read-only rootfs

Open-webOS-DCO-1.0-Signed-off-by: Iyad Qumei <iyad.qumei@lge.com>
Change-Id: Ie1f4402fdf46d2ef634033b62ca198453e6dfe7e
Reviewed-on: https://g2g.palm.com/2805
Reviewed-by: Build Verification
Reviewed-by: Iyad Qumei <iyad.qumei@lge.com>
Tested-by: Iyad Qumei <iyad.qumei@lge.com>
Reviewed-by: Ed Chejlava <ed.chejlava@lge.com>
Reviewed-by: Susan Montooth <susan.montooth@lge.com>