Ok, the PS.FI2 is need for the manual firmware update, the diskboot.bin is for auto booting chdk.
The encryption keys can be found in the ROM code or just ask someone in private.
For compilation from source,
I've renamed platform/fi2.inc.txt as told in the file itself, enabled the OPT_FI2 options, filled in the encryption keys
for the digic4 chip (the one my camera uses) and then compiled
export PATH=$PATH:/home/me/chdk/arm-elf/bin
make PLATFORM=sx200is PLATFORMSUB=100c fir
diskboot.bin and ps.fi2 showed up in the bin/ subdirectory.
