build: only specify least specific directory to dtbtool

New kernels (3.18) place dtb files into the qcom subdirectory,
but older kernels simply create a qcom subdirectory and place
the dtb files at the same level in the heirarchy.  Rather than
looking for the first directory that exists, dtbtool has been
modified to search all subdirectories for dtb files, and
therefore we have to remove the specific subdirectories.

Change-Id: I82e948c0a20605f051a60fdfaae9679e1fa6555e