buildroot

sepolgen: allow compiling for target

Sepolgen is currently a host-only package, however it is a dependency
of audit2allow.  This patch allows for sepolgen to be compiled for the
target.

Signed-off-by: Adam Duskett <Adamduskett@outlook.com>
Reviewed-by: Matt Weber <matthew.weber@rockwellcollins.com>
[Thomas:
 - fix formatting of Config.in file
 - add missing dependencies inherited from the fact that the package
   selects python3.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>