A f0nt is a binary compressed bitmap font.
f0n files can be created with makefont.pl - which requires perl and the GD module.
To convert a ttf font file, call it as “makefont.pl -b -f file.ttf -s 18” which should create (amongst others) a .F0N file.
Warning: if you load a broken font, your r0ket wont boot anymore. in this case you need to build and flash your own firmware to work around it