= 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.
