The argument, "type", is not used effectively in create_frame(). In
addition, it's confusing because create() calls create_frame() with
FRAME802154_DATAFRAME as "type" though it does not always create a data
frame.
tsch_packet_create_eb() is changed to build an Enhanced Beacon in the
packetbuf directly. Along with this, its signature is updated by
removing buf and buf_size from the argument list.
tsch_packet_create_eack() prepares a frame802154_t data by itself, which
is similar to what framer_802154_setup_params() does. In order to avoid
duplicate code, tsch_packet_create_eack() is going to use
framer_802154_setup_params() to have a ready-to-use frame802154_t data.
