As BlueZ 5 introduced backwards incompatible API changes, and we want to
keep support for BlueZ 4 based plugins for some time, we need to separate
the plugins that are based on BlueZ 4 from the ones based on BlueZ 5.
The bluetooth.c plugin has now been renamed to bluez4.c and the hfp_hf.c
plugin is renamed to hfp_hf_bluez4. This will make it easy to add a
HFP HF plugin for BlueZ 5.