Bluetooth: Move out string related function of `common/log.h`

Functions related to string manipulation that were defined in
`common/log.h` has been moved to the `common/bt_str.h` file and their
implementation in `common/bt_str.c`.

Files that were using those functions has been updated consequently.

Signed-off-by: Théo Battrel <theo.battrel@nordicsemi.no>