m68k: m68328: remove duplicate code

Building a kernel with multiple dragonball based boards
enabled needlessly causes a link failure because of duplicate
config_BSP() functions between the CPU versions.

Avoid that merging the three almost identical files into one,
and hiding the CPU configuration behind the board config.
The pr_info() lines are consolidated in one place.

It is still not possible to run a kernel configured for
more than one board, but at least configurations that can
be selected can also be built now.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>