upd: native Makefile improvements

- Remove optimization. We don't need it, and -O3 in particular can
  introduce bugs.
- Move -I directives to preprocessor flags.
- Add a separate goal for the header file instead of using shell &&.
- Use the special variable $(RM) to remove files, and ignore failures if
  some files don't exist.

Change-Id: Icb7bd684bae6bdb860712a4e24d880b265db292a