hcu4, hcu5: fix out of tree building

Out of tree building of the Netstal hcu4 and hcu5 boards failed like
that:

Assembler messages:
Fatal error: can't create /work/wd/tmp-ppc/board/netstal/hcu4/../common/fixed_sdram.o: No such file or directory
Assembler messages:
Fatal error: can't create /work/wd/tmp-ppc/board/netstal/hcu4/../common/nm_bsp.o: No such file or directory
make[1]: *** [/work/wd/tmp-ppc/board/netstal/hcu4/../common/fixed_sdram.o] Error 2

Adapt (and simplify) the respective Makefiles.

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