First apply board includes, then det. CC caps

In #664 I added a test that determines if the supplied compiler
understands the `-fno-delete-null-pointer-checks` flag. The problem is
that the `$(CC)` supplied on command line or in the application's
Makefile is used, but not the one the `$(BOARD)`'s Makefile sets.

That problem was overlooked as all the boards use GCC, and GCC happens
to know the flag. But if some future board does not use GCC, then the
wrong order of the checks could pose a problem.