[dtbTool] Fix version detection and find dtb in subfolders

With a simple fix seems to work fine.

This fixes the compilation of linux-t2m-flame and the generated dt.img is exactly the same as the one generated with the old dtbtool.

I'm compiling the linux-xiaomi-ido right now but I don't have the dt.img generated with the previous dtbTool to compare with

Close: #1055 (closed)