If you do damage your boot loader somehow (maybe put it in a microwave) then yes you can upload new bootloader with a ISP and set the appropriate fuses (boot loader size for eg)
Most common real bricking happen when you change the fuses to change the oscillator multiplier resulting in non standard baud rates.
This is still recoverable, but you have to hack on an appropriate oscillator get you the real baudrates again, this could take some guessing if you dont know what was set
But I repeat, you cant brick it via the firmware bootloader. Only a ISP has the ability to change data in the boot area.
No matter what mess of binary firmware you may upload, the boot loader will still be fine and run on next board reboot.
Most common real bricking happen when you change the fuses to change the oscillator multiplier resulting in non standard baud rates.
This is still recoverable, but you have to hack on an appropriate oscillator get you the real baudrates again, this could take some guessing if you dont know what was set
But I repeat, you cant brick it via the firmware bootloader. Only a ISP has the ability to change data in the boot area.
No matter what mess of binary firmware you may upload, the boot loader will still be fine and run on next board reboot.