[LEDE-DEV] include/kernel: Allow to use git clone kernel into external tree

[Giuseppe Lippolis]

> This combines the usage of external kernel tree with the cloning of the
> kernel tree form git. The new kernel will be cloned into an external
> directory and then used from there.

I find it very useful if someone need to develop a kernel patch.
If the problem is the download time my suggestion is enabling this function
as an option (non by default).

Bye.