checkpolicy: allow compiling for target

checkpolicy: allow compiling for target

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

Signed-off-by: Adam Duskett <Adamduskett@outlook.com>
Reviewed-by: Matt Weber <matthew.weber@rockwellcollins.com>
[Thomas:
 - remove LIBSELINUX_INSTALL_STAGING = YES, doing it in
   checkpolicy.mk is wrong, and libselinux is already installed to
   staging
 - add "select BR2_PACKAGE_LIBSELINUX" in Config.in, and propagate the
   necessary dependencies
 - add host-flex in dependencies, since it is also needed (in addition
   to target flex).]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>