kernel: Ensure dtb(o) always get rebuilt

 * AOSP build system is not aware of dtb(o) source update, and thus
   won't rebuild the images, results in outdated artifact.
 * Convert DTB(O)_OUT to make targets and make the images depend on them
   to address the issue. This makes AOSP build system be aware of
   their timestamp changes and re-execute recipes depending on them.
   Finally, it's up to kernel's build system to decide actions to take.

Test: m dtb(o)image and observe dtb(o).img get rebuilt
Change-Id: I197e7d2d3e013ded0e555fc22bb6a1200d40df9c