linux: add automatic selinux config selection

linux: add automatic selinux config selection

Enabling SELinux support in the kernel requires several options, many
of which are in different areas. These options are as follows:

  - CONFIG_AUDIT
  - CONFIG_DEFAULT_SECURITY_SELINUX
  - CONFIG_INET
  - CONFIG_NET
  - CONFIG_SECURITY
  - CONFIG_SECURITY_NETWORK
  - CONFIG_SECURITY_SELINUX

As such, if a user selects the libselinux package, it is much easier
to select these options for them, much like we already do with other
packages such as systemd or iptables.

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