If CFLAGS contains something like `-fdebug-prefix-map=/build/path=.`, we
need to remove it from CONFIG_ARGS so it doesn't get baked into `show
version`. Otherwise, build becomes non-reproducible if the build path
changes. To avoid other things creeping in, let's just remove *FLAGS in
their entirety. (Not really reliable information anyway.)
With this commit, FRR build should be 100% reproducible.
Signed-off-by: David Lamparter <equinox@diac24.net>