Move Ubuntu specific use_tempaddr setting to ubuntu-common element

By default (during boot) the use_tempaddr is set to <=0 for all
up-to date kernels.  Only Ubuntu installes a sysctl setting which
sets the use_tempaddr to 2 (/etc/sysctl.d/10-ipv6-privacy.conf) [1].
The 80-disable-rfc3041 overwrites this setting and sets
use_tempaddr back to 0.

Because this only affects Ubuntu it makes sense to move the script
to the ubuntu-common element. The other motivation for the move is,
to clear the base element that it can be removed.

[1] https://bugs.launchpad.net/ubuntu/+source/procps/+bug/1068756

Change-Id: Ibf261818ca8243874fde9eb3650bb65188fa228d
Signed-off-by: Andreas Florath <andreas@florath.net>

elements/ubuntu-common/README.rst

@@ -0,0 +1,5 @@
+ubuntu-common
+=============
+
+This element holds configuration and scripts that are common for all
+Ubuntu images.

elements/ubuntu-common/install.d/80-disable-rfc3041

@@ -21,7 +21,17 @@ if [ ${DIB_DEBUG_TRACE:-0} -gt 0 ]; then
 fi
 set -e
 
-# This will disable the disable Privacy extensions for IPv6 (RFC3041)
+# This will disable the privacy extensions for IPv6 (RFC4941)
+# (RFC4941 obsoletes RFC3041)
+#
+# Note that depending on the implementation of how the sysctl
+# settings are applied there is a race condition: there might be
+# a small time-frame where the original Ubuntu settings from
+# /etc/sysctl.d/10-ipv6-privacy.conf are already applied but the
+# new settings from /etc/sysctl.d/99-cloudimg-ipv6.conf
+# not yet.
+# If during this time all started network interfaces will
+# use the privacy extension.
 cat > /etc/sysctl.d/99-cloudimg-ipv6.conf <<EOF
 # See https://bugs.launchpad.net/ubuntu/+source/procps/+bug/1068756
 net.ipv6.conf.all.use_tempaddr=0

elements/ubuntu-core/element-deps

@@ -1,3 +1,4 @@
 cache-url
 dib-run-parts
 dpkg
+ubuntu-common

elements/ubuntu-minimal/element-deps

@@ -1,2 +1,3 @@
 debootstrap
 package-installs
+ubuntu-common

elements/ubuntu/element-deps

@@ -3,3 +3,4 @@ cloud-init-datasources
 dib-run-parts
 dkms
 dpkg
+ubuntu-common