package/chartjs: new package

Note: even though the Chart.js developers make specific tarballs on GitHub
(i.e. not simply 'source code' tarballs), they cannot be used in Buildroot
because their names do not encode a version number, e.g. 'Chart.js.zip'.
This means that on upgrades, the same tarball name would have different
contents and thus a different hash.

Signed-off-by: Joeri Barbarien <joeri.barbarien@nokia.com>
Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>