Add the systemd-bootchart package. At this stage just adding the
package. The plan is to add the module and kernel configurations for the
boot invocation in a follow up PR.
Being utilized in nixosConfigurations as part of Ghaf, where the
systemd-analyze does not provide enough information, especially cpu and
mem usages for throttling the start of some of the virtual machines.
This has been tested locally in realtime with:
```
/nix/store/...-systemd-bootchart-235/lib/systemd/systemd-bootchart --rel -o $(mktemp -d)
```
Waiting the requisite 20 seconds and then viewing the created .svg file.
Signed-off-by: Brian McGillion <bmg.avoin@gmail.com>
