FR: Provide Out-Of-Tree Building; Provide Cross-Compile Parameters

Let me know if I should be submitting each of these feature-requests separately.

For projects I built, up to the git project (think LFS steps (altered)), they've all provided OOT builds so the source directory remains pristine (albeit those that needed autoconf to be run for `configure` generation).

They've also all provided `--{build,host,target,with-sysroot}=` parameters for ease of cross-compiling.

Looking at what the git project offers, and I'm not seeing either features.

Looking at the Makefile, I see mention of `HOST_CPU` which looks pertinent, but being able to point to the path of the target root for files to link is important and I'm not noticing anything for that.

Grepping through Documentation for `HOST_CPU` comes up with nothing.