Fixing toolchain executable not found error for build.py
Fixing toolchain executable not found error for build.py

Commit 19d56fd40fc3130b970e03fd530ab340d4a65465 removed the default file
paths for the toolchains. This was done under the assumption that the
top-level compile scripts were properly checking that the toolchain
executable was availble. The build.py script was doing this in the wrong
place. This commit rearranges the script a bit so the check is performed
properly.

RonEld/mbed,netzimme/mbed-os,maximmbed/mbed,catiedev/mbed-os,mikaleppanen/mbed-os,Archcady/mbed-os,j-greffe/mbed-os,nRFMesh/mbed-os,adamgreen/mbed,kl-cruz/mbed-os,mazimkhan/mbed-os,YarivCol/mbed-os,mikaleppanen/mbed-os,pradeep-gr/mbed-os5-onsemi,mmorenobarm/mbed-os,fahhem/mbed-os,svogl/mbed-os,adamgreen/mbed,infinnovation/mbed-os,theotherjimmy/mbed,fahhem/mbed-os,maximmbed/mbed,HeadsUpDisplayInc/mbed,c1728p9/mbed-os,mikaleppanen/mbed-os,fahhem/mbed-os,adustm/mbed,betzw/mbed-os,ryankurte/mbed-os,mikaleppanen/mbed-os,arostm/mbed-os,HeadsUpDisplayInc/mbed,YarivCol/mbed-os,mazimkhan/mbed-os,monkiineko/mbed-os,c1728p9/mbed-os,monkiineko/mbed-os,arostm/mbed-os,arostm/mbed-os,pradeep-gr/mbed-os5-onsemi,NXPmicro/mbed,mmorenobarm/mbed-os,bcostm/mbed-os,infinnovation/mbed-os,kjbracey-arm/mbed,mazimkhan/mbed-os,adamgreen/mbed,arostm/mbed-os,cvtsi2sd/mbed-os,svogl/mbed-os,catiedev/mbed-os,arostm/mbed-os,betzw/mbed-os,netzimme/mbed-os,ryankurte/mbed-os,HeadsUpDisplayInc/mbed,fanghuaqi/mbed,j-greffe/mbed-os,CalSol/mbed,kl-cruz/mbed-os,cvtsi2sd/mbed-os,screamerbg/mbed,nRFMesh/mbed-os,karsev/mbed-os,screamerbg/mbed,RonEld/mbed,theotherjimmy/mbed,bulislaw/mbed-os,cvtsi2sd/mbed-os,catiedev/mbed-os,Archcady/mbed-os,NXPmicro/mbed,nRFMesh/mbed-os,pradeep-gr/mbed-os5-onsemi,adamgreen/mbed,nvlsianpu/mbed,NXPmicro/mbed,monkiineko/mbed-os,CalSol/mbed,betzw/mbed-os,pradeep-gr/mbed-os5-onsemi,radhika-raghavendran/mbed-os5.1-onsemi,netzimme/mbed-os,HeadsUpDisplayInc/mbed,radhika-raghavendran/mbed-os5.1-onsemi,mbedmicro/mbed,bulislaw/mbed-os,theotherjimmy/mbed,radhika-raghavendran/mbed-os5.1-onsemi,catiedev/mbed-os,maximmbed/mbed,screamerbg/mbed,YarivCol/mbed-os,svogl/mbed-os,nRFMesh/mbed-os,radhika-raghavendran/mbed-os5.1-onsemi,adamgreen/mbed,screamerbg/mbed,maximmbed/mbed,bcostm/mbed-os,fanghuaqi/mbed,betzw/mbed-os,kjbracey-arm/mbed,NXPmicro/mbed,adustm/mbed,svogl/mbed-os,infinnovation/mbed-os,fanghuaqi/mbed,andcor02/mbed-os,Archcady/mbed-os,radhika-raghavendran/mbed-os5.1-onsemi,nvlsianpu/mbed,theotherjimmy/mbed,CalSol/mbed,HeadsUpDisplayInc/mbed,j-greffe/mbed-os,svogl/mbed-os,andcor02/mbed-os,nvlsianpu/mbed,c1728p9/mbed-os,karsev/mbed-os,ryankurte/mbed-os,karsev/mbed-os,YarivCol/mbed-os,adustm/mbed,cvtsi2sd/mbed-os,mmorenobarm/mbed-os,j-greffe/mbed-os,mazimkhan/mbed-os,adustm/mbed,pradeep-gr/mbed-os5-onsemi,bcostm/mbed-os,fahhem/mbed-os,kl-cruz/mbed-os,fahhem/mbed-os,mbedmicro/mbed,kl-cruz/mbed-os,bcostm/mbed-os,c1728p9/mbed-os,kjbracey-arm/mbed,arostm/mbed-os,nRFMesh/mbed-os,kl-cruz/mbed-os,theotherjimmy/mbed,bulislaw/mbed-os,mikaleppanen/mbed-os,nRFMesh/mbed-os,bcostm/mbed-os,bulislaw/mbed-os,j-greffe/mbed-os,maximmbed/mbed,kl-cruz/mbed-os,j-greffe/mbed-os,CalSol/mbed,RonEld/mbed,infinnovation/mbed-os,andcor02/mbed-os,andcor02/mbed-os,fahhem/mbed-os,karsev/mbed-os,svogl/mbed-os,netzimme/mbed-os,monkiineko/mbed-os,ryankurte/mbed-os,bcostm/mbed-os,Archcady/mbed-os,YarivCol/mbed-os,mikaleppanen/mbed-os,YarivCol/mbed-os,cvtsi2sd/mbed-os,RonEld/mbed,betzw/mbed-os,catiedev/mbed-os,adustm/mbed,HeadsUpDisplayInc/mbed,netzimme/mbed-os,mmorenobarm/mbed-os,CalSol/mbed,mmorenobarm/mbed-os,RonEld/mbed,nvlsianpu/mbed,bulislaw/mbed-os,ryankurte/mbed-os,RonEld/mbed,maximmbed/mbed,fanghuaqi/mbed,kjbracey-arm/mbed,cvtsi2sd/mbed-os,theotherjimmy/mbed,mbedmicro/mbed,fanghuaqi/mbed,andcor02/mbed-os,mmorenobarm/mbed-os,karsev/mbed-os,screamerbg/mbed,betzw/mbed-os,screamerbg/mbed,adustm/mbed,pradeep-gr/mbed-os5-onsemi,monkiineko/mbed-os,mazimkhan/mbed-os,nvlsianpu/mbed,catiedev/mbed-os,c1728p9/mbed-os,mbedmicro/mbed,CalSol/mbed,radhika-raghavendran/mbed-os5.1-onsemi,infinnovation/mbed-os,andcor02/mbed-os,karsev/mbed-os,adamgreen/mbed,netzimme/mbed-os,monkiineko/mbed-os,Archcady/mbed-os,mazimkhan/mbed-os,ryankurte/mbed-os,bulislaw/mbed-os,NXPmicro/mbed,infinnovation/mbed-os,c1728p9/mbed-os,Archcady/mbed-os,mbedmicro/mbed,nvlsianpu/mbed,NXPmicro/mbed