build manuals moved here from osmo-gsm-manuals.git

Moved to doc/manuals/, with full commit history, in preceding merge commit.
Now incorporate in the build system.

Build with:

$ autoreconf -fi
$ ./configure --enable-manuals
$ make

Shared files from osmo-gsm-manuals.git are found automatically if
- the repository is checked out in ../osmo-gsm-manuals; or
- if it osmo-gsm-manuals was installed with "make install"; or
- OSMO_GSM_MANUALS_DIR is set.

Related: OS#3385
Change-Id: I2762171af0bf719a34ba12a0c2e4dcc206098beb
diff --git a/doc/manuals/osmotrx-vty-reference.xml b/doc/manuals/osmotrx-vty-reference.xml
index ae0afbf..4291929 100644
--- a/doc/manuals/osmotrx-vty-reference.xml
+++ b/doc/manuals/osmotrx-vty-reference.xml
@@ -5,7 +5,7 @@
 -->
 <!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML 5.0//EN"
 "http://docbook.org/xml/5.0/dtd/docbook.dtd" [
-<!ENTITY chapter-vty      SYSTEM      "../common/chapters/vty.xml" >
+<!ENTITY chapter-vty      SYSTEM      "./common/chapters/vty.xml" >
 <!ENTITY sections-vty     SYSTEM      "generated/docbook_vty.xml"  >
 ]>