doc: add info on libyang install and configure options

Signed-off-by: Lou Berger <lberger@labn.net>

doc/developer/building-libyang.rst

@@ -4,6 +4,23 @@ The libyang library can be installed from third-party packages available `here
 Note: the libyang dev/devel packages need to be installed in addition
 to the libyang core package in order to build FRR successfully.
 
+For example, for CentOS 7.x:
+
+.. code-block:: shell
+
+   wget https://ci1.netdef.org/artifact/LIBYANG-YANGRELEASE/shared/build-1/CentOS-7-x86_64-Packages/libyang-0.16.46-0.x86_64.rpm
+   wget https://ci1.netdef.org/artifact/LIBYANG-YANGRELEASE/shared/build-1/CentOS-7-x86_64-Packages/libyang-devel-0.16.46-0.x86_64.rpm
+   sudo rpm -i libyang-0.16.46-0.x86_64.rpm libyang-devel-0.16.46-0.x86_64.rpm
+
+or Ubuntu 18.04:
+
+.. code-block:: shell
+
+   wget https://ci1.netdef.org/artifact/LIBYANG-YANGRELEASE/shared/build-1/Ubuntu-18.04-x86_64-Packages/libyang-dev_0.16.46_amd64.deb
+   wget https://ci1.netdef.org/artifact/LIBYANG-YANGRELEASE/shared/build-1/Ubuntu-18.04-x86_64-Packages/libyang_0.16.46_amd64.deb
+   sudo apt install libpcre3-dev
+   sudo dpkg -i libyang-dev_0.16.46_amd64.deb libyang_0.16.46_amd64.deb
+
 Alternatively, libyang can be built and installed manually by following
 the steps below:
 
@@ -23,3 +40,23 @@ When building libyang on CentOS 6, it's also necessary to pass the
 Note: please check the `libyang build requirements
 <https://github.com/CESNET/libyang/blob/master/README.md#build-requirements>`_
 first.
+
+Libyang uses loadable libraries an YANG modules.  It supports
+environment variables to allow overriding the load paths for each of
+these. With FRR, this override currently must be done at the time of
+running FRR's configure command using new options. The new options are:
+
+.. code-block:: shell
+
+   --with-yangmodelsdir=DIR
+                          yang models directory (${datarootdir}/yang)
+   --with-libyang-pluginsdir=DIR
+                          yangmodule plugins directory
+                          (${libdir}/frr/libyang_plugins)
+
+an example which uses the compile directory is:
+
+.. code-block:: shell
+
+   ./configure --with-libyang-pluginsdir="`pwd`/yang/libyang_plugins/.libs" \
+          --with-yangmodelsdir="`pwd`/yang"