It gets better...
Leave platform.txt alone and add these lines to boards.txt instead.
menu.printf=AVR printf Version
mega.menu.printf.default=Default printf
mega.menu.printf.default.compiler.c.elf.extra_flags=
mega.menu.printf.full=Full printf
mega.menu.printf.full.compiler.c.elf.extra_flags=-Wl,-u,vfprintf -lprintf_flt
mega.menu.printf.minimal=Minimal printf
mega.menu.printf.minimal.compiler.c.elf.extra_flags=-Wl,-u,vfprintf -lprintf_min
Caveat: has to be duplicated for each board.