manuals: generate vty reference xml at build time

Remove ggsn_vty_reference from the source tree.

In manuals/Makefile.am use the new BUILT_REFERENCE_XML feature recently added
to osmo-gsm-manuals, and add a build target to generate the XML using the new
osmo-ggsn --vty-ref-xml cmdline switch.

Change-Id: I772293cc78a6c95e07565a7048c1c8dadf87d2fc
Depends: I613d692328050a036d05b49a436ab495fc2087ba
Related: OS#5041

doc/manuals/Makefile.am

@@ -11,6 +11,12 @@ if BUILD_MANUALS
   include $(OSMO_GSM_MANUALS_DIR)/build/Makefile.asciidoc.inc
 
   VTY_REFERENCE = osmoggsn-vty-reference.xml
+
+  BUILT_REFERENCE_XML = $(builddir)/vty/ggsn_vty_reference.xml
+  $(builddir)/vty/ggsn_vty_reference.xml: $(top_builddir)/ggsn/osmo-ggsn
+	mkdir -p $(builddir)/vty
+	$(top_builddir)/ggsn/osmo-ggsn --vty-ref-xml > $@
+
   include $(OSMO_GSM_MANUALS_DIR)/build/Makefile.vty-reference.inc
 
   OSMO_REPOSITORY=osmo-ggsn