nios2: build .dtb files in dts directory

Align nios2 with other architectures which build the dtb files in the
same directory as the dts files. This is also in line with most other
build targets which are located in the same directory as the source.
This move will help enable the 'dtbs' target which builds all the dtbs
regardless of kernel config.

This transition could break some scripts if they expect dtb files in
the old location.

Cc: Ley Foon Tan <lftan@altera.com>
Cc: nios2-dev@lists.rocketboards.org
Signed-off-by: Rob Herring <robh@kernel.org>