build-tools: set the prefix before loading the config
build-tools: set the prefix before loading the config

This ensures that all the environment variable set by the
platform config file are correctly set using the build-tools
prefix

EricssonResearch/cerbero,brion/cerbero,shoreflyer/cerbero,flexVDI/cerbero,justinjoy/cerbero,GStreamer/cerbero,shoreflyer/cerbero,nzjrs/cerbero,davibe/cerbero,nzjrs/cerbero,lubosz/cerbero,freedesktop-unofficial-mirror/gstreamer__sdk__cerbero,nirbheek/cerbero,multipath-rtp/cerbero,sdroege/cerbero,superdump/cerbero,nirbheek/cerbero-old,nirbheek/cerbero,jackjansen/cerbero,atsushieno/cerbero,OptoFidelity/cerbero,centricular/cerbero,EricssonResearch/cerbero,ford-prefect/cerbero,freedesktop-unofficial-mirror/gstreamer__cerbero,brion/cerbero,flexVDI/cerbero,freedesktop-unofficial-mirror/gstreamer-sdk__cerbero,davibe/cerbero,ylatuya/cerbero,ikonst/cerbero,nirbheek/cerbero,EricssonResearch/cerbero,nzjrs/cerbero,ikonst/cerbero,ramaxlo/cerbero,BigBrother-International/gst-cerbero,sdroege/cerbero,ford-prefect/cerbero,shoreflyer/cerbero,ramaxlo/cerbero,freedesktop-unofficial-mirror/gstreamer-sdk__cerbero,lubosz/cerbero,cee1/cerbero-mac,sdroege/cerbero,BigBrother-International/gst-cerbero,atsushieno/cerbero,nirbheek/cerbero-old,OptoFidelity/cerbero,cee1/cerbero-mac,ylatuya/cerbero,AlertMe/cerbero,jackjansen/cerbero,multipath-rtp/cerbero,centricular/cerbero,atsushieno/cerbero,freedesktop-unofficial-mirror/gstreamer__sdk__cerbero,ford-prefect/cerbero,ylatuya/cerbero,brion/cerbero,superdump/cerbero,nirbheek/cerbero-old,ford-prefect/cerbero,shoreflyer/cerbero,BigBrother-International/gst-cerbero,sdroege/cerbero,superdump/cerbero,BigBrother-International/gst-cerbero,jackjansen/cerbero-2013,nzjrs/cerbero,multipath-rtp/cerbero,centricular/cerbero,jackjansen/cerbero,lubosz/cerbero,freedesktop-unofficial-mirror/gstreamer__sdk__cerbero,ikonst/cerbero,flexVDI/cerbero,shoreflyer/cerbero,ikonst/cerbero,centricular/cerbero,centricular/cerbero,GStreamer/cerbero,justinjoy/cerbero,OptoFidelity/cerbero,jackjansen/cerbero-2013,ramaxlo/cerbero,EricssonResearch/cerbero,freedesktop-unofficial-mirror/gstreamer-sdk__cerbero,GStreamer/cerbero,freedesktop-unofficial-mirror/gstreamer__cerbero,GStreamer/cerbero,lubosz/cerbero,multipath-rtp/cerbero,fluendo/cerbero,atsushieno/cerbero,jackjansen/cerbero-2013,fluendo/cerbero,fluendo/cerbero,AlertMe/cerbero,multipath-rtp/cerbero,fluendo/cerbero,brion/cerbero,nirbheek/cerbero-old,AlertMe/cerbero,davibe/cerbero,ikonst/cerbero,nzjrs/cerbero,GStreamer/cerbero,superdump/cerbero,EricssonResearch/cerbero,cee1/cerbero-mac,freedesktop-unofficial-mirror/gstreamer__cerbero,ramaxlo/cerbero,freedesktop-unofficial-mirror/gstreamer__cerbero,cee1/cerbero-mac,ylatuya/cerbero,davibe/cerbero,sdroege/cerbero,AlertMe/cerbero,jackjansen/cerbero,fluendo/cerbero,nirbheek/cerbero,jackjansen/cerbero-2013,flexVDI/cerbero,OptoFidelity/cerbero,freedesktop-unofficial-mirror/gstreamer__sdk__cerbero,BigBrother-International/gst-cerbero,flexVDI/cerbero,freedesktop-unofficial-mirror/gstreamer-sdk__cerbero,justinjoy/cerbero,AlertMe/cerbero,jackjansen/cerbero-2013,atsushieno/cerbero,justinjoy/cerbero,freedesktop-unofficial-mirror/gstreamer-sdk__cerbero,ramaxlo/cerbero,brion/cerbero