Build gem5 through buildroot

Oh, btw, also make it work, since the build was completely broken since
the recent -K refactoring :0)

gem5/.gitignore

@@ -1 +1 @@
-/gem5-system
+/system

gem5/README.adoc

@@ -0,0 +1,10 @@
+= gem5
+
+We Build gem5 as a Buildroot package for the following reasons:
+
+* if we build in tree without Buildroot, we still want to build the `m5` tool through Buildroot. But then Buildroot rsyncs the build for all archs into `output/build`, which takes quite some disk space. There seems to be no way to exclude some packages from the sync.
+* it allows us to reuse Buildroot's:
+** configuration system, which reduces duplication: set the gem5 package option, everything just gets built
+** timestamping system, which could save a few seconds on rebuilds, since we then don't ever run `scons` when gem5 is not modified
+
+What is preventing upstreaming to Buildroot: a minimal kernel config like the ones QEMU has.

gem5/build

@@ -0,0 +1,45 @@
+#/usr/bin/env bash
+set -eu
+arch=x86_64
+j=''
+while getopts a:j: OPT; do
+  case "$OPT" in
+    a)
+      arch="$OPTARG"
+      ;;
+    j)
+      j="$OPTARG"
+      ;;
+  esac
+done
+shift "$(($OPTIND - 1))"
+if [ -z "$j" ]; then
+  j="$(nproc)"
+fi
+top="$(dirname "$0")"
+system_dir="${top}/system"
+binaries_dir="${system_dir}/binaries"
+disks_dir="${system_dir}/disks"
+mkdir -p "$binaries_dir" "$disks_dir"
+cd "${top}/gem5"
+if [ "$arch" = x86_64 ]; then
+  CC=gcc-6 CXX=g++-6 scons -j"$j" --ignore-style build/X86/gem5.opt
+  f="${disks_dir}/linux-bigswap2.img"
+  dd if=/dev/zero of="$f" bs=1024 count=65536
+  mkswap "$f"
+  # This file must always be present, despite --kernel overriding that default and selecting the kernel.
+  # I'm not even joking. No one has ever built x86 gem5 without the magic dist dir present.
+  touch "${binaries_dir}/x86_64-vmlinux-2.6.22.9"
+elif [ "$arch" = arm ] || [ "$arch" = aarch64 ]; then
+  # Compilation fails with gcc 7 on that commit.
+  # There were some recent portability patches, so it will likely get there soon.
+  CC=gcc-6 CXX=g++-6 scons -j"$j" --ignore-style build/ARM/gem5.opt
+  make -C ./system/arm/dt
+  # TODO use the buildroot cross compiler here, and remove the dependencies from configure.
+  make -C ./system/arm/simple_bootloader
+  make -C ./system/arm/aarch64_bootloader/
+  cp ./system/arm/simple_bootloader/boot_emm.arm "$binaries_dir"
+  cp ./system/arm/aarch64_bootloader/boot_emm.arm64 "$binaries_dir"
+fi
+# TODO vs telnet?
+make -C util/term

gem5/external.mk

@@ -15,6 +15,7 @@ ARCH_MAKE = $(ARCH)
 endif
 
 define GEM5_BUILD_CMDS
+	cd '$(@D)' && ./build -a '$(ARCH)' -j '$(BR2_JLEVEL)'
 	cd '$(@D)/gem5/util/m5' && $(MAKE) -f 'Makefile.$(ARCH_MAKE)' CC='$(TARGET_CC)' LD='$(TARGET_LD)'
 endef