Fix bug where private prebuilts are uploaded with https://.
Fix bug where private prebuilts are uploaded with https://.

If upload_prebuilts is called with --private but not with --board, the
script currently sets https:// as the binhost base prefix, which is
incorrect. Fix this.

BUG=chromium:242570
TEST=Unit tests.
TEST=cbuildbot --remote --buildbot --debug alex-chrome-pfq
TEST=cbuildbot --remote --buildbot --debug x86-generic-chromium-pfq

Change-Id: I31d981b69bdcfa9ba834ba83e4add426a0ce9a95
Reviewed-on: https://gerrit.chromium.org/gerrit/56057
Reviewed-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Commit-Queue: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Tested-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>

bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite