Return pointer when using bt_get()

bt_get() now returns the pointer it was passed to replace code
of the form:

bt_get(my_thingy);
another_object->thingy = my_thingy;

to the shorter form

another_object->thingy = bt_get(my_thingy);

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>