[yocto] examples / docs on utilizing an external toolchain

[Kumar Gala]

It seems like there is a way to use a prebuilt toolchain with poky but no real details.

Some refs in the docs like:

POKYMODE
Toolchain selector. It can be external toolchain built from Poky or few supported combinations of upstream GCC or CodeSourcery Labs toolchain.

But grepping the code there doesn't seem to be any actual use of POKYMODE.  There seems to be some references even on the autobuilder about this 'nightly-external-toolchain'.  So wondering what the details where on how to configure things to use an external toolchain.

- k