[Buildroot] [PATCH 3/9] package/libredblack: add package

[Heiko Thiery]

> The Github page says "
> Copy of RedBlack Balanced Tree Searching and Sorting Library -
> http://libredblack.sourceforge.net/". Does it make sense to use the
> original version from SourceForge instead ?

While taking a look into that we figured out that a (maybe more
common) lib called libavl can also be used for building the sysrepo.

Therefore now we will provide a package for that libavl instead of the
libredblack. This libavl is also used in debian so we think it is
better maintained.