Hello HyungJong,
I'm beginning to wonder if the problem you are seeing lies with the configuration of etherboot you've used.
My thinking is that etherboot thinks it's executing a different type of image (ie FreeBSD, Multiboot, etc) rather than an ELF image, and is thus passing some different information across.
Can you forward the Config file that you used to build the etherboot payload ?
Regards Mark Wilkinson.