manuals: generate vty reference xml at build time

Move 'doc' subdir further down to "make sure" the osmo-mgw binary is built
before the docs.

Remove mgw_vty_reference.xml 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-mgw --vty-ref-xml cmdline switch.

Depends: I613d692328050a036d05b49a436ab495fc2087ba (osmo-gsm-manuals)
Change-Id: I526af21134087e2b43b9ada59c93f636ae242e24
diff --git a/doc/manuals/Makefile.am b/doc/manuals/Makefile.am
index a82f95d..7242015 100644
--- a/doc/manuals/Makefile.am
+++ b/doc/manuals/Makefile.am
@@ -13,6 +13,11 @@
   VTY_REFERENCE = osmomgw-vty-reference.xml
   include $(OSMO_GSM_MANUALS_DIR)/build/Makefile.vty-reference.inc
 
+  BUILT_REFERENCE_XML = $(builddir)/vty/mgw_vty_reference.xml
+  $(builddir)/vty/mgw_vty_reference.xml: $(top_builddir)/src/osmo-mgw/osmo-mgw
+	mkdir -p $(builddir)/vty
+	$(top_builddir)/src/osmo-mgw/osmo-mgw --vty-ref-xml > $@
+
   OSMO_REPOSITORY = osmo-mgw
   include $(OSMO_GSM_MANUALS_DIR)/build/Makefile.common.inc
 endif