initrd-setup-root: Add selective OS reset handling

A reconfiguration of a system will lead to config drift is the local
state isn't discarded. However, while this is possible with Ignition
reformatting the rootfs, this big hammer is too disruptive.
Introduce a way to perform a selective OS reset where a special file
contains a regex for paths to preserve. This regex should be prepared
by a helper tool like
  flatcar-reset --keep-paths "/etc/ssh/" "/var/docker.*"
that would assemble the regex to be
  (/etc/ssh|/etc/ssh/.*|/var/docker.*)
where the omitted slash at the end of the /etc/ssh/ is needed due to
how "find" prints the paths, and the additional entry ssh/.* for the
contents of /etc/ssh/ which would otherwise be deleted - but we should
allow the user to use a final slash anyway instead of forcing the user
to add /.* everywhere, thus the helper tool should take care of it. If
the user wants to remove all contents but keep the empty dir,
^/etc/ssh or /etc/ssh$ or (/etc/ssh) or /etc/(ssh) would work to
directly specify the wanted regex without the helper tool expanding it.

dracut/99setup-root/initrd-setup-root

@@ -31,6 +31,34 @@ function walksysroot() {
 COREOS_BLANK_MACHINE_ID="42000000000000000000000000000042"
 MACHINE_ID_FILE="/sysroot/etc/machine-id"
 
+function selectiveosreset() {
+  local entry="/sysroot$1"
+  # Don't remove /sysroot itself
+  [ "${entry}" = "/sysroot" ] && return 0
+  [ "${entry}" = "/sysroot/" ] && return 0
+  # Don't remove the active /usr mount point
+  [ "${entry}" = "/sysroot/usr" ] && return 0
+  # Not really needed because find doesn't add a trailing slash but to be safe:
+  [ "${entry}" = "/sysroot/usr/" ] && return 0
+  if [ -d "${entry}" ]; then
+    # Try to delete dir, will fail if its contents are preserved
+    usrbin rmdir "${entry}" 2>/dev/null || true
+  else
+    rm "${entry}" || true # TODO: Report through flag file or fail the boot?
+  fi
+  true # Do not carry any last condition evaluation over as return code
+}
+
+# Do the selective OS reset as prepared by flatcar-reset
+if [ -s /sysroot/selective-os-reset ]; then
+  walksysroot / selectiveosreset -regextype egrep -not -regex "$(cat /sysroot/selective-os-reset)"
+  rm -f /sysroot/selective-os-reset
+  # Always remove the machine-id file because otherwise it's not a first boot.
+  # The previous value can be preserved through the systemd.machine_id=
+  # kernel parameter.
+  rm -f /sysroot/etc/machine-id
+fi
+
 # This creates the modifiable users/groups in /sysroot/etc,
 # initializing the shadow database in the process. This needs to
 # happen early, so systemd-tmpfiles can read the user info from