ISP with 115200 baud
You have to edit the programmers.txt file. (The one in ~/.arduino15/packages/arduino/hardware/avr/1.6.21/, not the one in arduino-1.8.x/hardware/arduino/avr/)

avrisp.name=AVR ISP
avrisp.communication=serial
avrisp.protocol=stk500v1
avrisp.program.protocol=stk500v1
avrisp.program.speed=115200
avrisp.program.tool=avrdude
avrisp.program.extra_params=-P{serial.port} -b{program.speed}


Add a avrisp.program.speed option, and pass this option as one of the extra_params to the command line call of avrdude.
It is then used on line 112 of platform.txt:

tools.avrdude.program.pattern="{cmd.path}" "-C{config.path}" {program.verbose} {program.verify} -p{build.mcu} -c{protocol} {program.extra_params} "-Uflash:w:{build.path}/{build.project_name}.hex:i"


You could of course also make a copy of that avrisp entry, and add your own programmer, just give it a different name.