Do the build.

        The target can set build_mode to 'release' or 'debug' before calling
        this method.

        (:meth:`prepare_for_build` must have been call before.)

def build(self):
        '''Do the build.

        The target can set build_mode to 'release' or 'debug' before calling
        this method.

        (:meth:`prepare_for_build` must have been call before.)
        '''
        assert(self.target is not None)
        assert(hasattr(self.target, '_build_prepared'))

        if hasattr(self.target, '_build_done'):
            return

        # increment the build number
        self.build_id = int(self.state.get('cache.build_id', '0')) + 1
        self.state['cache.build_id'] = str(self.build_id)

        self.info('Build the application #{}'.format(self.build_id))
        self.build_application()

        self.info('Package the application')
        self.target.build_package()

        # flag to prevent multiple build
        self.target._build_done = True