Some configuration files that need updates are directly under in /etc. To
update them atomically, we need write access to /etc. For Ubuntu Core this is
an issue as /etc is not writable. Only a selection of subdirectories can be
writable. The general solution is symlinks or bind mounts to writable places.
But for atomic writes in /etc, that does not work. So Ubuntu has had a patch
for that that did not age well.
Instead we would like to introduce some environment variables for alternate
paths.
* SYSTEMD_ETC_HOSTNAME: /etc/hostname
* SYSTEMD_ETC_MACHINE_INFO: /etc/machine-info
* SYSTEMD_ETC_LOCALTIME: /etc/localtime
* SYSTEMD_ETC_LOCALE_CONF: /etc/locale.conf
* SYSTEMD_ETC_VCONSOLE_CONF: /etc/vconsole.conf
* SYSTEMD_ETC_ADJTIME: /etc/adjtime
While it is for now expected that there is a symlink from the standard, we
still try to read them from that alternate path. This is important for
`/etc/localtime`, which is a symlink, so we cannot have an indirect symlink or
bind mount for it.
Since machine-id is typically written only once and not updated. This commit
does not cover it. An initrd can properly create it and bind mount it.
0dc39dffbd
