Feature request: when building kernels, output to separate directory to minimize the number of patches needed

Most Android downstream kernels are built with make O=(separate output directory).

postmarketOS's kernel template builds kernels inside the source directory directly.

A kernel build that outputs to a different directory has slightly different include paths than a build that outputs inside the same directory.

This means that many downstream kernels require patches to #include statements to compile under pmOS. eg this one for crosshatch.

To minimize the number of patches required, we should modify our kernel templates to support building in a separate directory, if needed by a specific kernel.