Add support for [blubber] section in Blubberfile
Add support for 'build' command

This command can be used to bitbake a project specific default target,
which is defined by setting a DEFAULT_BUILD in the [blubber] section of
the Blubberfile.
Add a [blubber] section to the minimal Blubberfile that is created
with the 'create' command.
Add poky master to default Blubberfile