Bluetooth: Initial conversion to net_buf API

This patch performs the minimal necessary conversion to the net_buf
API. It uses a temporary "#define bt_buf net_buf" to make it possible
to convert the code in smaller increments. Most old bt_buf function
also serve as one-line wrappers to the matching net_buf APIs. Once
everything is converted these helpers will be removed.

Change-Id: Ie31433d33576022c9c193a35d2389267005545d6
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>