Is it possible to configure gpio from level of the bootloader? I need to set GPIO to output low level before my application is loaded. I know that simple hardware pull-down would be the easiest way, but for some reason in our procject it is not an option.
The SDK only provides access to application level code. You can toggle the GPIO before the rest of the application code is executed. But not before that point in time. If you try to do it before some BLE initialization functions, you'll probably find that some of the hardware hasn't yet been initialized and your code may crash. Be sure to initialize any necessary hardware.