[Buildroot] External toolchain support improvements

[Shinya Kuribayashi <skuribay@ruby.dti.ne.jp>]

Peter Korsgaard wrote:
> No, these are settings for the external toolchain, E.G. configuration
> options you have to set to match whatever your external toolchain has.

Sure.

> Don't get misled by the fact that we have the same configuration
> variables (with different help text) for the normal buildroot
> toolchain.

I know that, of course.

This patch (and probably your comments) seems to overlook toolchains
built with _multi libraries_.  Please do not assume toolchain's default
options or default configurations, especially when we talk about
external toolchain support.

Thanks,

  Shinya