webos-image: Pull opkg with package-management IMAGE_FEATURE

* package-management IMAGE_FEATURE pulls always right implementation
  based on selected PACKAGE_CLASS
  deb: run-postinsts dpkg apt
  ipk: opkg opkg-collateral ${EXTRAOPKGCONFIG}
  rpm: rpm zypper (rpm smartpm in newer oe-core)
* also this IMAGE_FEATURE prevents removing package manager database
  in do_rootfs

Open-webOS-DCO-1.0-Signed-off-by: Martin Jansa <martin.jansa@palm.com>
Change-Id: I0f6403ae4b283fb87f50cc2c692dcdeff2df8eb5
Reviewed-on: http://g2g.palm.com/1375
Reviewed-by: Build Verification
Reviewed-by: Martin Jansa <martin.jansa@palm.com>
Tested-by: Martin Jansa <martin.jansa@palm.com>
Reviewed-by: Susan Montooth <susan.montooth@palm.com>