enabling W=1 by default

Hi!

looking at the redness of the output of `make W=1` here is the question: isn't it a good time to enable `make W=1` by default. Yes, I understand the impact, but at least we can do it mandatory for a _new_ code submitted to U-Boot, right?

Ideally I would have what Linux kernel has for a few releases already, i.e. Werror by default and getting close to make a clean builds with that and make W=1` at least against default configurations (yeah, with U-Boot there is probably no default, but sandbox one).