package/genpart: make it a target package, too

genpart comes handy when there is a need to programatically generate
partition tables, without the need for a full partitioning program like
fdisk et al.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>