base-files: allow reusing of boolean value extraction logic

base-files: allow reusing of boolean value extraction logic

The `functions.sh` script has `config_get_bool()` function, which is
usable when using UCI config direct access API, but there is no
equivalent for the callback API. Introduce `get_bool()` function to
allow reusing it from init scripts.

Example:

```sh
option_cb() {
    local option="$1"
    local value="$(get_bool "$2")"
    ...
}
```

Signed-off-by: Oldřich Jedlička <oldium.pro@gmail.com>