debian: create -doc subpackage with pdf manuals

I have verified, that the resulting debian packages build in my own OBS
namespace (see the -doc packages):
https://download.opensuse.org/repositories/home:/osmith42/Debian_9.0/all/
https://build.opensuse.org/project/show/home:osmith42

Depends: Ib7251cca9116151e473798879375cd5eb48ff3ad (osmo-ci)
Related: OS#3899
Change-Id: I34858a18a34fc467f274ac164697a242f4cf0df8
diff --git a/debian/control b/debian/control
index ffb9ac1..8ff59f0 100644
--- a/debian/control
+++ b/debian/control
@@ -14,7 +14,8 @@
                libtalloc-dev,
                libusrp-dev,
                liblimesuite-dev,
-               libosmocore-dev (>= 0.10.0)
+               libosmocore-dev (>= 0.10.0),
+               osmo-gsm-manuals-dev
 Standards-Version: 3.9.6
 Vcs-Browser: http://cgit.osmocom.org/osmo-trx
 Vcs-Git: git://git.osmocom.org/osmo-trx
@@ -89,3 +90,12 @@
  3GPP is the "3rd Generation Partnership Project" which is the collaboration
  between different telecommunication associations for developing new
  generations of mobile phone networks. (post-2G/GSM)
+
+Package: osmo-trx-doc
+Architecture: all
+Section: doc
+Priority: optional
+Depends: ${misc:Depends}
+Description: ${misc:Package} PDF documentation
+ Various manuals: user manual, VTY reference manual and/or
+ protocol/interface manuals.