arm: Remove support for smdk6400

The migration of boards from Makefile to boards.cfg was due for v2012.03, but
smdk6400 did not follow, and it does not build, so move it to scrapyard. It will
still be possible to restore it from the Git history before fixing it.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>