scripts: Add build-standalone-img to build images used without capstan

This script is similar to build-capstan-img. Later we can hook this
script to scripts/build-osv-release to build images used without
capstan. It takes the same args with build-capstan-img.

Example output:

   $ find build/standalone
   build/standalone/cloudius/
   build/standalone/cloudius/osv-iperf
   build/standalone/cloudius/osv-iperf/osv-iperf.esx.ova
   build/standalone/cloudius/osv-iperf/osv-iperf.qemu.qcow2
   build/standalone/cloudius/osv-iperf/osv-iperf.vbox.ova
   build/standalone/cloudius/osv-iperf/osv-iperf.vmw.zip
   build/standalone/cloudius/osv-iperf/osv-iperf.gce.tar.gz

Signed-off-by: Asias He <asias@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
build-osv-release: standalone image support

Make scripts/build-osv-release support both Capstan and standalone
images.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
Revert "build: Skip rebuild of usr.img on osv.vmdk and osv.vdi target"

This reverts commit df30ec8d32dac5ad72d71314ab327c48d4e8a054. It breaks
building "osv.vdi" altogether.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>