Bluetooth: Kconfig: Rename CONFIG_BLUETOOTH_* to CONFIG_BT_*

The API name space for Bluetooth is bt_* and BT_* so it makes sense to
align the Kconfig name space with this. The additional benefit is that
this also makes the names shorter. It is also in line with what Linux
uses for Bluetooth Kconfig entries.

Some Bluetooth-related Networking Kconfig defines are renamed as well
in order to be consistent, such as NET_L2_BLUETOOTH.

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