Bluetooth: Define global Bluetooth address constants

The Bluetooth address constants (BT_ADDR[_LE]_ANY, BT_ADDR[_LE]_NONE)
are currently defined as the address of the local anonymous structs
that are initialised to the corresponding address values, and assigning
them to a variable whose scope is greater than that of a function may
end up creating dangling pointers (for instance, as done in the
`bt_conn_get_info` function).

This commit defines the Bluetooth address constants as global constant
variables that are placed in the read-only data section, and modifies
the Bluetooth address constant macros to use the address of these
variables instead.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>