Otherwise, adoc files in osmo-*/doc/manuals/chapters/*.adoc cannot
include files from osmo-gsm-manuals/common/chapters/*.adoc, since
symbolic link to common/ is only available in builddir, but not in
srcdir.
This way, those files can include commons files like this:
include::{commondir}/chapters/foobar.adoc
Change-Id: Ifc2fdaf1a87d0715213b4eab13c6c9a4f8a987fd