Revert "We don't hame hammerhead-kernel repository.."

This reverts commit dad4e3138eb27afad019903056d280f50c15bb00.
Revert "Use prebuilt kernel instead of building zImage"

This reverts commit 285d3c28c78d67cab30462563c3c0af6c7452b80.
use default toolchain since we're building stock kernel source
F2FS

Signed-off-by: kykdev <kykint@naver.com>