memtest86+: Allow configuration to be defined externally

- Only set the #defines in config.h that aren't already defined.  This
allows them to be overridden by the makefile.
- Allow serial options to be set in the makefile

Change-Id: I02362f6960ee7bc169911e77cb85f2f02fb2aed2
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/13827
Reviewed-by: Ben Gardner <gardner.ben@gmail.com>
2 files changed