Removing default toolchain paths
Removing default toolchain paths

This change was spurred by a confusing error. I attempted to compile for
the RZ_A1H (a Cortex-A device), and I had the standalone ARM compiler in
my system path, which supports Cortex-A. However, the default path for
the ARM compiler in settings.py uses a Keil installation, which only
supports Cortex-M. It found my Keil installation and used that instead.
This change proposes to remove this default behavior and instead
requires the user to explicitly set the intended compiler, either by a
settings file, mbed CLI, environment variables, or by placing the
compiler in your PATH.

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