Disabled boost download when boost is already downloaded.

Disabled boost download when boost is already downloaded.

This effectively disables the build server from downloading boost
when it already has it. (Before it was making its decision on
whether or not to download it based solely on whether or not the
*.zip file already existed.)

Change-Id: I155fec0428b7caa0b517bbbe6458ee85e692ec8b
Signed-off-by: Joseph Morrow <joseph.l.morrow@intel.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/1768
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Erich Keane <erich.keane@intel.com>