libyang: New dependency for frr build

libyang: New dependency for frr build

- Impementation of libyang-2.0.7 as a dependency for the build of frr
- Creation of rootfile with all entries commented out so that it is only used for the build
   libyang is a YANG data modelling language parser and toolkit written (and providing API)
   in C.In the future if there is demand to use these functions in frr then this package
   may need to be moved from a build only option to a dependency for frr providing the
   yang libraries.
- Added into make.sh just before frr

Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Arne Fitzenreiter <arne_f@ipfire.org>