mcu25: fix out of tree building

Out of tree building of the Netstal mcu25 board failed like
that:

Configuring for mcu25 board...
Assembler messages: Fatal error: can't create /work/wd/tmp-ppc/board/netstal/mcu25/../common/fixed_sdram.o: No such file or directory
Assembler messages: Fatal error: can't create /work/wd/tmp-ppc/board/netstal/mcu25/../common/nm_bsp.o: No such file or directory

Adapt (and simplify) the Makefile.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Niklaus Giger <niklaus.giger@netstal.com>