Packaging:  fix broken `install` recipe

GNU Make's built-in shell is old-school Bourne shell, and doesn't
recognize the bash `foo-{bar,baz}`-style wildcards.  (This simple fact
seems to be a steep learning curve for YT.)  If this had worked, it
would break package builds by removing the installed files list for
`xenomai-kernel` threads.

Also, use `rm -f` and remove the initial `-` from statements:

  - `rm -f` doesn't fail if the file doesn't exist, but does fail if
    the file exists but can't be removed.  The latter case is a true
    error that shouldn't be suppressed.

  - The copy of debian/extras had also better succeed.