Makefile.include (clean): ignore rm -f failing (e.g., on "core")

rm -f can still fail, e.g., if trying to delete a directory.

If there was, say, a directory called "core", a "make clean" would
therefore only try to delete the files listed in the first command but
not proceed with the rest of the cleanup.

"make clean" itself failing may also affect any outside build process
that invokes it.
Merge pull request #418 from frtos-wpan/uninterrupted-cleanup

Makefile.include (clean): ignore rm -f failing (e.g., on "core")
Merge remote-tracking branch 'upstream/master' into uninterrupted-cleanup