manuals: generate vty reference xml at build time

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

Rremove sgsn_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-sgsn --vty-ref-xml cmdline switch.

Change-Id: Ied75fdd13283aa592350994fb4febce06c213d3a
Depends: I613d692328050a036d05b49a436ab495fc2087ba
diff --git a/Makefile.am b/Makefile.am
index 17b3fc6..023a470 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -9,11 +9,11 @@
 	$(NULL)
 
 SUBDIRS = \
-	doc \
 	include \
 	src \
 	contrib \
 	tests \
+	doc \
 	$(NULL)
 
 BUILT_SOURCES = $(top_srcdir)/.version