kbuild: avoid race between dtbs and dt/dt.dtb targets

If the final targets depend on both "dtbs" and "dts/dt.dtb",
and -j option is given to the command line, multiple threads
descend into the dts/ directory, which causes build error.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Tested-by: Andreas Dannenberg <dannenberg@ti.com>