kernel: Add support to append DTBs to kernel image

* Many Qualcomm targets that uses boot header v0 or v1
  have the DTBs appended to the end of kernel image.

* In downstream msm kernels up to msm-4.19, Appending DTBs
  is handled in the kernel tree. However, on msm-5.4 and
  newer msm kernels, and on mainline kernel, This is not
  supported.

* Normally, DTB processing depends on how the bootloader
  loads it and should not be handled in the kernel tree,
  so we handle it here.

* Instead of appending all DTBs by default, Get the list of
  DTBs that needs to be included from a variable, so that
  we don't need to modify dts Makefile to exclude unwanted
  DTBs, and we could also ensure the DTBs are appended in
  a specific order.

Change-Id: I603d1ebac5ee808bad045b85f9868d572d52cb80