systemd-bootchart: relicense to LGPLv2.1+

When we merged systemd-bootchart we omitted relicensing this
to LGPL. Now that code is being used for other purposes we
need to make sure the code is shareable within systemd
without further problems.

The original committers and contributors of bootchart before it
was merged all have agreed with the relicensing.
Merge branch 'master' of ssh://git.freedesktop.org/git/systemd/systemd into work