fix build command always rebuild payloads files

    def select_payloads(self):

        '''

        Return a generator on image payloads

        if tobuild is True, return only needed to build

        '''

        for payname in os.listdir(self.payload_path):

            yield payname

    def create_payloads(self):

        '''

        Create all data payloads in current directory

        Create all missing data payloads in current directory

        Doesn't compute md5 during creation because tarball can

        be created manually

        Also create symlink to versionned payload

        '''

        arrow("Creating payloads")

        for payload_name in self.select_payloads():

            paydesc = self.describe_payload(payload_name)

            if os.path.exists(paydesc["link_path"]):

                continue

            arrow(payload_name, 1)

            try:

                # create non versionned payload file

                if not os.path.exists(paydesc["dest_path"]):

                    if paydesc["isdir"]:

                        self.create_payload_tarball(paydesc["dest_path"],

                                                    paydesc["source_path"])

                    else:

                        self.create_payload_file(paydesc["dest_path"],

                                                 paydesc["source_path"])

                # create versionned payload file

                if os.path.lexists(paydesc["link_path"]):

                    os.unlink(paydesc["link_path"])

                os.symlink(paydesc["dest_path"], paydesc["link_path"])

            except Exception as e:

                raise Exception("Unable to create payload %s: %s" % (payload_name, e))

    def create_payload_tarball(self, tar_path, data_path):

        '''

        Create a payload tarball