feat(cros-kernel/x86_64_defconfig) add "root=gptprio:" to kernel command line

For places where you can't control the kernel boot command line (i.e.
UEFI), add a default option so that dracut knows what to do with
booting.
feat(cros-kernel/x86_64_defconfig) build USB into the kernel

When booting from a USB device, you need the USB drivers built into the
kernel.
Merge pull request #185 from gregkh/kernel_update

kernel config: build in command line option and usb drivers