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/

run

@@ -90,6 +90,7 @@ done
 shift "$(($OPTIND - 1))"
 extra_flags="$extra_flags $@"
 set_common_vars "$arch" "$gem5"
+images_dir="${buildroot_out_dir}/images"
 if "$debug" && "$kvm"; then
   echo 'error: -d and -K are incompatible' 1>&2
   exit 1
@@ -117,29 +118,27 @@ if [ -n "$extra_append_after_dash" ]; then
 fi
 
 if "$gem5"; then
-  build_dir="${out_dir}/build/gem5-1.0"
+  gem5_build_dir="${buildroot_out_dir}/build/gem5-1.0"
+  gem5_src_dir="${gem5_build_dir}/gem5"
   memory="${memory}B"
-  gem5_dir="${build_dir}/gem5"
   if [ "$arch" = x86_64 ]; then
     gem5_arch=X86
   else
     gem5_arch=ARM
   fi
-  m5out_dir="${root_dir}/m5out"
-  gem5_cpt_dir="${m5out_dir}/cpts/${arch}"
-  mkdir -p "$gem5_cpt_dir"
+  m5out_dir="${gem5_out_dir}/m5out"
   gem5_common="\
-M5_PATH='${build_dir}/system' \
-$debug_vm \
-'${gem5_dir}/build/${gem5_arch}/gem5.opt' \
+M5_PATH='${gem5_build_dir}/system' \
+${debug_vm} \
+'${gem5_src_dir}/build/${gem5_arch}/gem5.opt' \
 ${gem5opts} \
-'${gem5_dir}/configs/example/fs.py' \
---checkpoint-dir='${gem5_cpt_dir}' \
---disk-image='${out_dir}/images/rootfs.ext2' \
---kernel='${out_dir}/build/linux-custom/vmlinux' \
+-d '${m5out_dir}' \
+'${gem5_src_dir}/configs/example/fs.py' \
+--disk-image='${images_dir}/rootfs.ext2' \
+--kernel='${buildroot_out_dir}/build/linux-custom/vmlinux' \
 --mem-size=${memory} \
 --num-cpus='${cpus}' \
---script='${m5out_dir}/myreadfile' \
+--script='${root_dir}/readfile.gitignore' \
 "
   if [ "$arch" = x86_64 ]; then
     if "$kvm"; then
@@ -155,7 +154,7 @@ ${extra_flags} \
     # Anything smaller than physical blows up as expected, but why can't it auto-detect the right value?
     cmd="${gem5_common} \
 --command-line='earlyprintk=pl011,0x1c090000 console=ttyAMA0 lpj=19988480 rw loglevel=8 mem=${memory} root=/dev/sda ${extra_append}' \
---dtb-file='${gem5_dir}/system/arm/dt/$([ "$arch" = arm ] && echo "armv7_gem5_v1_${cpus}cpu" || echo "armv8_gem5_v1_${cpus}cpu").dtb' \
+--dtb-file='${gem5_src_dir}/system/arm/dt/$([ "$arch" = arm ] && echo "armv7_gem5_v1_${cpus}cpu" || echo "armv8_gem5_v1_${cpus}cpu").dtb' \
 --machine-type=VExpress_GEM5_V1 \
 ${extra_flags} \
 "
@@ -167,15 +166,15 @@ else
   extra_flags="${extra_flags_qemu} ${extra_flags}"
   qemu_common="\
 ${debug_vm} \
-'${out_dir}/host/usr/bin/qemu-system-${arch}' \
+'${buildroot_out_dir}/host/usr/bin/qemu-system-${arch}' \
 -gdb tcp::1234 \
 -m ${memory} \
 -monitor telnet::45454,server,nowait \
 -netdev user,hostfwd=tcp::45455-:45455,hostfwd=tcp::45456-:22,id=net0 \
 -smp ${cpus} \
--trace enable=${trace_enable},file='${lkmc_out_dir}/trace.bin' \
+-trace enable=${trace_enable},file='${qemu_out_dir}/trace.bin' \
 -virtfs local,path=9p,mount_tag=host_scratch,security_model=mapped,id=host_scratch \
--virtfs local,path=${out_dir}/build,mount_tag=host_out,security_model=mapped,id=host_out \
+-virtfs local,path=${buildroot_out_dir}/build,mount_tag=host_out,security_model=mapped,id=host_out \
 "
   if "$initrd"; then
     extra_flags="${extra_flags} -initrd '${images_dir}/rootfs.cpio'"