dtc: Optimise by default, fix warnings thus uncovered

[Jon Loeliger]

So, like, the other day David Gibson mumbled:
> This patch turns on optimisation in the Makefile by default.  With the
> optimizer on, some uninitialized variable warnings (one real, two
> bogus) are now generated.  This patch also squashes those again.

Applied.

Thanks,
jdl