Changes related to logging

This patch modifes:
  - mcpu.Processor.LogWarning to have its 'hint' parameter as optional
    and only log it if not None
  - cmdlib._WaitForSync to not log directly to stdout/stderr but via the
    proc.Log(Info|Warning) methods
  - the LU attribute 'processor' is renamed to 'proc' to shorten the
    name

Reviewed-by: imsnah