Delete .consign files generated by cons

These files are build products storing checksums so that cons
knows when to rebuild them, so it's not really of interest to
track them. Plus tracking them makes them show up as modified
in git status every time the project is rebuilt.

Add a .gitignore file that ignores the .consign files, plus
the temp_gccflags.c intermediate produced by cons, and the
".slXXX" directories where the build products are written.