dtc: Enable more warnings

[Jon Loeliger]

> This patch turns on a bunch of extra gcc warnings, most of which are
> probably a good idea.  Of the new warnings -Wnested-externs and
> -Wstrict-prototypes need no code changes, we're already warning-clean.
> The remaining one, -Wmissing-prototypes requires trivial changes in
> some of the tests (making functions local).
> 
> This patch also rearranges the warnings flags into a separate make
> variable for convenience, and turns on -Werror, to really encourage
> people to keep the code warning-clean.
> 
> Signed-off-by: David Gibson <david at gibson.dropbear.id.au>
> 
> ---
> 
> Third time lucky.  Please?

Applied!

jdl