Disable separated DTB build and remove dtbtool

* hammerhead uses DTB appended to zImage (zImage-dtb)

Change-Id: I938c0531130bc76a04494b13411cd95a6d960984
Signed-off-by: Vojtech Bocek <vbocek@gmail.com>
hammerhead: Set timer_rate for cpufreq governor

 * Reduces audio jitter

Change-Id: I0ee9c052ba07f0c5d5621cee868ce33fb68f7efc