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>
db6df604f7