Clean up chroot helpers

Removed obsolete chroot helpers on Disco+:
 - selinux
 - sysv-rc
 - upstart

Added a note about needlessly triggering chroot_archives on each pass.

live-build/lb_chroot_layered

@@ -129,17 +129,18 @@ create_chroot_pass() {
 	lb chroot_cache restore ${*}
 	lb chroot_devpts install ${*}
 	lb chroot_proc install ${*}
-	lb chroot_selinuxfs install ${*}
 	lb chroot_sysfs install ${*}
 	lb chroot_debianchroot install ${*}
 	lb chroot_dpkg install ${*}
 	lb chroot_tmpfs install ${*}
-	lb chroot_sysv-rc install ${*}
-	lb chroot_upstart install ${*}
 	lb chroot_hosts install ${*}
 	lb chroot_resolv install ${*}
 	lb chroot_hostname install ${*}
 	lb chroot_apt install ${*}
+	# Note: this triggers an upgrade + dist-ugprade; which may impact sublayers with more
+	# diff content than desired. However, we still need to setup the archive and teardown
+	# for each layer.
+	# We could modify livebuild if necessary to have conditional upgrade (first pass only).
 	lb chroot_archives chroot install ${*}
 
 	# Customizing chroot
@@ -186,13 +187,10 @@ create_chroot_pass() {
 	lb chroot_hostname remove ${*}
 	lb chroot_resolv remove ${*}
 	lb chroot_hosts remove ${*}
-	lb chroot_sysv-rc remove ${*}
-	lb chroot_upstart remove ${*}
 	lb chroot_tmpfs remove ${*}
 	lb chroot_dpkg remove ${*}
 	lb chroot_debianchroot remove ${*}
 	lb chroot_sysfs remove ${*}
-	lb chroot_selinuxfs remove ${*}
 	lb chroot_proc remove ${*}
 	lb chroot_devpts remove ${*}
 	lb chroot_cache save ${*}