m68k: remove arch/m68k/lib/board.c

All the M68000 boards have switched to Generic Board.
This file is no longer necessary.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Huan Wang <alison.wang@freescale.com>
Cc: Angelo Dureghello <angelo@sysam.it>
Reviewed-by: Simon Glass <sjg@chromium.org>