Go in /hardware/arduino/avr .
Create a file platform.local.txt and add this line in it:
compiler.c.elf.extra_flags= -Wl,-u,vfprintf -lprintf_flt -lm -Wl,-u,vfscanf -lscanf_flt -lm
Save file, and restart Arduino IDE. Now you should be able to use sprintf and sscanf with floats 
But be aware that this will use around 2KB of memory (around 1% of the available memory of an Arduino Mega 2560).