I'm sorry maeda to my knowledge there is no documentation available about poweroff-gpio.c. But I will do a little research to see if I can find some information about it, and if I do, I will post it here, to see if it helps you.
I checked and there is documentation on this, and actually it is included in the package, you can find it here: build/tmp/work/edison-poky-linux/linux-yocto/3.10.17+gitAUTOINC+6ad20f049a_c03195ed6e-r0/linux/Documentation/devicetree/binding/gpio/gpio-poweroff.txt
Peter, Thank you for information.
I think it is about very complex "Device tree".
My system requires GP47=L after power-off.
I think this is achieved by this text:
compatible = "gpio-poweroff";
gpios = <&gpio 47 0>;
But, Which file this text should be appended to?
What command needed before "bitbake edison-image"?
We can only point you to the basic documentations included with the gpio kernel driver that was provided with Yocto not Intel. I suggest you check the following file build/tmp/work/edison-poky-linux/linux-yocto/3.10.17+gitAUTOINC+6ad20f049a_c03195ed6e-r0/linux/Documentation/gpio.txt, study and get familiar with the gpio driver files in the folder build/tmp/work/edison-poky-linux/linux-yocto/3.10.17+gitAUTOINC+6ad20f049a_c03195ed6e-r0/linux/drivers/gpio, read books like "Embedded Linux Development with Yocto Project" http://www.amazon.com/gp/product/1783282339/ref=pe_375460_130605410_em_1p_0_ti, post your question to the Yocto forum, or contact the gpio-poweroff.c author, you can find his email at the end of the file gpio-poweroff.c.