package/libselinux: enable kernel selinux support for jffs2 images

If BR2_TARGET_ROOTFS_JFFS2 is selected, enable the following kernel options:
  - CONFIG_JFS_SECURITY

Signed-off-by: Adam Duskett <Aduskett@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>