bitbake-diffsigs: handle if task name is specified without do_ prefix

Adjust the task name automatically if the -t option is specified with
a task name that doesn't start with do_ (e.g. "configure" instead of
"do_configure").

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
bitbake-diffsigs: refactor argument parsing slightly

* Use OptionParser to parse the two options to -t rather than trying to
  pick them out ourselves.
* Add a description shown with --help output

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
bitbake-diffsigs: improve error handling

* Set up a logger independent of BitBake so we can log errors ourselves
* Handle common errors without printing a traceback

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
bitbake-dumpsig: introduce command line and error handling

This utility doesn't take any special arguments, but it's nice if it at
least knows how to deal with no arguments, --help and errors properly.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>