spinel: Make assert logging opt-in. (#876)
spinel: Make assert logging opt-in. (#876)

This change makes assert logging an opt-in proposition. By default,
this means that spinel will not make any calls to `printf` or `fprintf`
when it hits a non-fatal runtime assertion (like an input validation
failure).

This was causing some problems on some platforms.
lanyuwen/openthread,jjlee3/openthread,nibanks/openthread,jwhui/openthread,nibanks/openthread,librasungirl/openthread,jjlee9/openthread,georgecpr/openthread,pvanhorn/openthread,chshu/openthread,jwhui/openthread,JakubBrachTieto/openthread,mszczodrak/openthread,openthread/openthread,xiaom-GitHub/openthread,gandreello/openthread,nibanks/openthread,mszczodrak/openthread,turon/openthread,jjlee9/openthread,jjlee3/openthread,abtink/openthread,erja-gp/openthread,chshu/openthread,JakubBrachTieto/openthread,pvanhorn/openthread,fbsder/openthread,chshu/openthread,georgecpr/openthread,JakubBrachTieto/openthread,jjlee3/openthread,JakubBrachTieto/openthread,aeliot/openthread,Zolertia/openthread,srickardti/openthread,jjlee9/openthread,LeZhang2016/openthread,jwhui/openthread,openthread/openthread,openthread/openthread,turon/openthread,Zolertia/openthread,abtink/openthread,aeliot/openthread,GiedriusM/openthread,xiaom-GitHub/openthread,xiaom-GitHub/openthread,srickardti/openthread,srickardti/openthread,erja-gp/openthread,lanyuwen/openthread,LeZhang2016/openthread,Zolertia/openthread,Zolertia/openthread,georgecpr/openthread,vaas-krish/openthread,JakubBrachTieto/openthread,librasungirl/openthread,mszczodrak/openthread,vaas-krish/openthread,LeZhang2016/openthread,GiedriusM/openthread,gandreello/openthread,jjlee3/openthread,jjlee9/openthread,gandreello/openthread,bukepo/openthread,abtink/openthread,fbsder/openthread,librasungirl/openthread,fbsder/openthread,nibanks/openthread,vaas-krish/openthread,fbsder/openthread,librasungirl/openthread,xiaom-GitHub/openthread,fbsder/openthread,bukepo/openthread,nibanks/openthread,lanyuwen/openthread,lanyuwen/openthread,jwhui/openthread,pvanhorn/openthread,georgecpr/openthread,gandreello/openthread,LeZhang2016/openthread,chshu/openthread,jjlee3/openthread,vaas-krish/openthread,mszczodrak/openthread,erja-gp/openthread,vaas-krish/openthread,jjlee9/openthread,vaas-krish/openthread,erja-gp/openthread,GiedriusM/openthread,chshu/openthread,LeZhang2016/openthread,lanyuwen/openthread,srickardti/openthread,chshu/openthread,aeliot/openthread,fbsder/openthread,bukepo/openthread,pvanhorn/openthread,GiedriusM/openthread,GiedriusM/openthread,erja-gp/openthread,aeliot/openthread,turon/openthread,JakubBrachTieto/openthread,pvanhorn/openthread,gandreello/openthread,gandreello/openthread,jjlee9/openthread,Zolertia/openthread,openthread/openthread,mszczodrak/openthread,georgecpr/openthread,aeliot/openthread,bukepo/openthread,turon/openthread,abtink/openthread,nibanks/openthread,erja-gp/openthread,LeZhang2016/openthread,lanyuwen/openthread,xiaom-GitHub/openthread,mszczodrak/openthread,georgecpr/openthread,aeliot/openthread,xiaom-GitHub/openthread,pvanhorn/openthread