Enable file prefix map for Makefiles for reproducible builds

[puddly]

When switching from CMake back to Makefiles, the -ffile-prefix-map config was not copied over. This PR re-introduces it and allows for easily comparing changes:

% git diff --no-index --stat -w build1 build2
 {build1 => build2}/ncp-uart-hw.Makefile    | 6 +++---
 {build1 => build2}/ncp-uart-hw.project.mak | 2 +-
 2 files changed, 4 insertions(+), 4 deletions(-)