Stop referencing the crazy target/device stuff

We don't need Config.in and Makefile in target/device: defconfig files
are sufficient to describe the specificities of a board (architecture,
compilation flags, bootloader and kernel details, etc.).

However, a placeholder such as target/device will be kept in order to
host things such as kernel configuration files or various
board-specific patches.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>