hob: fix build again when building packages only

hob: fix build again when building packages only

The different code paths for package vs image builds have lead to some bit
rot in the package only build and an incompatability between package only
builds and the build again mechanism.

This patch unifies the code paths and fixes build again for package only
builds.

Fixes [YOCTO #1480]

(Bitbake rev: ab6c63f35739c30df0e8a9d0f1ae982b96e538ed)

Signed-off-by: Joshua Lock <josh@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>