Makefile/mkconfig: read simple board configurations from boards.cfg

Instead of adding explicit build rules for each and every board to the
top level Makefile (which makes it grow and grow), we now provide a
simple default rule and extend the "mkconfig" script to read board
configurations from a plain text file (table), "boards.cfg".

For simple boards it is now sufficient to add a single line of text to
the "boards.cfg" file, no changes to the top level Makefile are needed
any more.

To make the table better readable, change the notation for unused
fields from "NULL" into "-".

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Peter Tyser <ptyser@xes-inc.com>
Cc: Mike Frysinger <vapier@gentoo.org>