The .efi file is used to recover the Galileo firmware. You have to use it together with the .cap file. The process is explained here: Intel® Quark SoC — BSP Build Guide, in section 9.
You can also find a step-by-step guide in the following thread explaining how to perform the process to recover the firmware using these files: Re: Intel Galileo Gen 2 board bricked.
Let me know if this information has clarified how the .efi file can be used.