Omit more unnecessary files from rootfs.squashfs

These are related to making SELinux policy changes at runtime.

squashfs.exclude

@@ -10,6 +10,8 @@ etc/init.d
 etc/portage
 etc/python-exec
 etc/runlevels
+etc/selinux/semanage.conf
+etc/selinux/sepolgen.conf
 usr/aarch64-unknown-linux-gnu
 usr/bin/2to3*
 usr/bin/audit2*
@@ -45,6 +47,7 @@ usr/share/pkgconfig
 usr/share/polkit-1
 usr/share/portage
 usr/share/selinux/*/include
+usr/share/selinux/devel
 usr/share/zsh
 var/.updated
 var/cache/edb