Move all build and run output to out/$arch/

Most of it was present inside buildroot/output.* and the rest scattered
on top level.

This came about for the n-th time when we were reviewing QEMU trace file
locations.

On one hand, it would be cool to have per arch traces.

This made buildroot/output.${arch}~/ a natural choice.

But on the other, those traces have nothing to do with Buildroot,
and could potentially interfere with Buildroot build files.

It also feels nicer to have buildroot/ pristine source code only,
and keep all output under a single directory out/

common

@@ -8,8 +8,12 @@ set_common_vars() {
   if "$gem5" && [ ! "$arch" = aarch64 ]; then
     arch_dir="${arch}-gem5"
   fi
-  out_dir="${buildroot_dir}/output.${arch_dir}~"
-  config_file="${out_dir}/.config"
-  lkmc_out_dir="${out_dir}/lkmc"
-  images_dir="${out_dir}/images"
+  out_dir="${root_dir}/out"
+  out_arch_dir="${out_dir}/${arch_dir}"
+  buildroot_out_dir="${out_arch_dir}/buildroot"
+  build_dir="${buildroot_out_dir}/build"
+  host_dir="${buildroot_out_dir}/host"
+  gem5_out_dir="${out_arch_dir}/gem5"
+  qemu_out_dir="${out_arch_dir}/qemu"
+  common_dir="${out_dir}/common"
 }