add CONFIG to skip ATAG passed devtree blob

On the older Motorola QCOM phones we're enabling DTB APPEND
for loading from the end of the kernel binary so that the
devtree partition doesn't need to be changed.

Change-Id: I7b62b9ec08d4f99c57547aa4d4e7b69b3f512bcb
variant defconfig: xt926

Change-Id: Ied60fabd3b18b288563e0b366dcdf5be3a357633