deploy-license-manifest: fix deploy_license_manifest failure

Rather than assuming we know where this logic needs to go (manually adding it
to the image postprocess commands), append directly to the license.bbclass
function which produces the manifest.

Signed-off-by: Christopher Larson <kergoth@gmail.com>
Merge pull request #93 from kergoth/vmware-vm-license-manifest-failure

deploy-license-manifest: fix deploy_license_manifest failure