libeigen: initial contribution required by opencv

As opencv depends on libeigen, libeigen is added to the meta-oe
layer. This commit is derived from the work [1] by Kartik Mohta.

[1] https://github.com/kartikmohta/meta-km/tree/master/recipes-extended/eigen

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@oss.bmw-carit.de>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>