My guess as an engineer is.....
The software is stored in non-valatile flash memory. Flash memory does not go away when the power is off. The drawback to flash memory is that is is extremely slow compared to modern RAM memory. It also has a limit to the number of writes over its lifetime, so it's not something that you want run an OS off of. There is likely some small programmable logic device in the radio, that once power is applied, reads the software from flash and writes it into RAM. Because when power goes away, so does all of the contents of RAM.
If I am correct, then I wouldn't worry too much about your boot loader. It's either going to work or not. And if I am correct, and the boot loader is a programmable logic device, its device configuration firmware will also be stored in flash memory. Should that ever get corrupted, your radio is as good as a brick. They'd be crazy to allow the user to update that.