cpu/intel/fsp_model_206ax: change realpath to readlink

realpath and readlink can be used to do the same thing - in this case
we're turning path1/path2/../path3/path4 into path1/path3/path4 so
that the makefile's wildcard routine can evaluate it.

Debian derivatives don't seem to include realpath. (and even when it's
installed, it's not the gnu coreutils version.)

Change-Id: I0a80a1d9b563810bdf96aea9d5de79ce1cea457a
Signed-off-by: Martin Roth <gaumless@gmail.com>
Reviewed-on: http://review.coreboot.org/5793
Reviewed-by: Marc Jones <marc.jones@se-eng.com>
Tested-by: build bot (Jenkins)