Apple_Boot "Boot OS X" partition and Fusion Drive
If the partition is there, but the files are corrupted, just remove them, boot to a rescue system or installer, open a shell and run touch /Volumes/Macintosh\ HD/System/Library/Extensions or kextcache -i /Volumes/Macintosh\ HD, with Macintosh HD being the name of your Fusion Drive/the mount point. After a while (or during shutdown), the booter should be regenerated.