bluez 5: Build both headset backends, if available

Enable both ofono and native backends to be built into the same
libbluez5-util. Never build the null backend.

Signed-off-by: David Henningsson <david.henningsson@canonical.com>
bluez 5: Load the native headset backend if the oFono one is unavailable

This implements some autodetect if both headset backends are compiled in:
First we try to contact the oFono service, if that's not working,
then we start the native backend instead.

Likewise if the oFono service is going offline/online, we load/unload
the native backend accordingly.

Signed-off-by: David Henningsson <david.henningsson@canonical.com>
bluez 5: Fix a debug message

Signed-off-by: David Henningsson <david.henningsson@canonical.com>
bluez 5: remove null headset backend

There is no need to have a "null" backend anymore.

Signed-off-by: David Henningsson <david.henningsson@canonical.com>
bluetooth: Select headset backend through module argument

This patch adds a module argument "headset=ofono|native|auto" to
module-bluetooth-discover and module-bluez5-discover.

To make Arun's happy, the default is 'native' if compiled in, otherwise
'ofono'. 'Auto' will try to autoswitch depending on whether ofono is
running or not.

Signed-off-by: David Henningsson <david.henningsson@canonical.com>