Make it possible to set log file

And use stdout for CC3200 demo because
  (1) an apparent bug which causes output sent to stderr to be
      printed v-e-r-y s-l-o-w-l-y (yes, it is really like that;
      https://e2e.ti.com/support/development_tools/code_composer_studio/f/81/t/501881)
  (2) in CCS it is printed in BLOOD RED, which is annoying

PUBLISHED_FROM=36a86744bc8ea193e99e98670dadc7f3ab6ed53e