If the bootloader is closed source, where would you put this #define? Write it with a pencil on the back of the chip? :-)
Luckily it's Open Source: [github.com] There are other Open Source bootloaders for the same purpose, another commonly used one is optiboot.
Also, bootloader baud rate is not to be confused with firmware baud rate. These two can have different baud rates. During firmware runtime, the bootloader isn't used at all, firmware comes with its own serial I/O code (and a #define to select it).
Luckily it's Open Source: [github.com] There are other Open Source bootloaders for the same purpose, another commonly used one is optiboot.
Also, bootloader baud rate is not to be confused with firmware baud rate. These two can have different baud rates. During firmware runtime, the bootloader isn't used at all, firmware comes with its own serial I/O code (and a #define to select it).