bootstrap: override build-tools config variables correctly
bootstrap: override build-tools config variables correctly

nzjrs/cerbero,lubosz/cerbero,ford-prefect/cerbero,atsushieno/cerbero,ramaxlo/cerbero,cee1/cerbero-mac,lubosz/cerbero,ikonst/cerbero,EricssonResearch/cerbero,atsushieno/cerbero,freedesktop-unofficial-mirror/gstreamer-sdk__cerbero,atsushieno/cerbero,nirbheek/cerbero,ylatuya/cerbero,AlertMe/cerbero,shoreflyer/cerbero,justinjoy/cerbero,nirbheek/cerbero-old,davibe/cerbero,lubosz/cerbero,brion/cerbero,ramaxlo/cerbero,sdroege/cerbero,fluendo/cerbero,freedesktop-unofficial-mirror/gstreamer__sdk__cerbero,centricular/cerbero,freedesktop-unofficial-mirror/gstreamer-sdk__cerbero,justinjoy/cerbero,centricular/cerbero,atsushieno/cerbero,OptoFidelity/cerbero,cee1/cerbero-mac,GStreamer/cerbero,GStreamer/cerbero,fluendo/cerbero,superdump/cerbero,BigBrother-International/gst-cerbero,brion/cerbero,nzjrs/cerbero,ford-prefect/cerbero,fluendo/cerbero,ikonst/cerbero,multipath-rtp/cerbero,nzjrs/cerbero,cee1/cerbero-mac,flexVDI/cerbero,AlertMe/cerbero,BigBrother-International/gst-cerbero,freedesktop-unofficial-mirror/gstreamer-sdk__cerbero,jackjansen/cerbero,ikonst/cerbero,OptoFidelity/cerbero,ramaxlo/cerbero,flexVDI/cerbero,multipath-rtp/cerbero,sdroege/cerbero,ylatuya/cerbero,freedesktop-unofficial-mirror/gstreamer__cerbero,BigBrother-International/gst-cerbero,superdump/cerbero,flexVDI/cerbero,freedesktop-unofficial-mirror/gstreamer__cerbero,BigBrother-International/gst-cerbero,BigBrother-International/gst-cerbero,ylatuya/cerbero,fluendo/cerbero,shoreflyer/cerbero,nirbheek/cerbero-old,shoreflyer/cerbero,justinjoy/cerbero,brion/cerbero,centricular/cerbero,multipath-rtp/cerbero,jackjansen/cerbero,ramaxlo/cerbero,EricssonResearch/cerbero,freedesktop-unofficial-mirror/gstreamer__sdk__cerbero,freedesktop-unofficial-mirror/gstreamer-sdk__cerbero,sdroege/cerbero,superdump/cerbero,AlertMe/cerbero,OptoFidelity/cerbero,ramaxlo/cerbero,ikonst/cerbero,OptoFidelity/cerbero,flexVDI/cerbero,superdump/cerbero,jackjansen/cerbero,davibe/cerbero,freedesktop-unofficial-mirror/gstreamer-sdk__cerbero,nzjrs/cerbero,ikonst/cerbero,davibe/cerbero,cee1/cerbero-mac,nirbheek/cerbero,multipath-rtp/cerbero,freedesktop-unofficial-mirror/gstreamer__cerbero,fluendo/cerbero,nirbheek/cerbero,lubosz/cerbero,davibe/cerbero,justinjoy/cerbero,sdroege/cerbero,nzjrs/cerbero,GStreamer/cerbero,ford-prefect/cerbero,jackjansen/cerbero,EricssonResearch/cerbero,atsushieno/cerbero,ford-prefect/cerbero,centricular/cerbero,brion/cerbero,freedesktop-unofficial-mirror/gstreamer__cerbero,freedesktop-unofficial-mirror/gstreamer__sdk__cerbero,nirbheek/cerbero-old,GStreamer/cerbero,freedesktop-unofficial-mirror/gstreamer__sdk__cerbero,multipath-rtp/cerbero,flexVDI/cerbero,nirbheek/cerbero-old,sdroege/cerbero,AlertMe/cerbero,AlertMe/cerbero,brion/cerbero,shoreflyer/cerbero,centricular/cerbero,shoreflyer/cerbero,EricssonResearch/cerbero,nirbheek/cerbero,EricssonResearch/cerbero,GStreamer/cerbero,ylatuya/cerbero