Support absolute path in chromiumsrcdir

Use the second parameter of $$absolute_path to resolve the
git config value qtwebengine.chromiumsrcdir, in case it's already
absolute.

Change-Id: I68caf5c2f208d3caae69c1f6f361c916ce3f3d93
Reviewed-by: Zeno Albisser <zeno.albisser@digia.com>
Reviewed-by: Andras Becsi <andras.becsi@digia.com>