debug-generator: add a kernel cmdline option to pause the boot process

Introduce the `systemd.break=` kernel command line option to allow stopping the
boot process at a certain point and spawn a debug shell. After exiting this
shell, the system will resume booting.

It accepts the following values:
- `pre-udev`: before starting to process kernel uevents (initrd and host).
- `pre-basic`: before leaving early boot and regular services start (initrd and
host).
- `pre-mount`: before the root filesystem is mounted (initrd).
- `pre-switch-root`: before switching root (initrd).

man/kernel-command-line.xml

@@ -97,6 +97,18 @@
         </listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><varname>systemd.break=</varname></term>
+        <term><varname>rd.systemd.break=</varname></term>
+        <listitem>
+          <para>Parameters understood by
+          <citerefentry><refentrytitle>systemd-debug-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
+          to pause the boot process at a certain point and spawn a debug shell.</para>
+
+          <xi:include href="version-info.xml" xpointer="v258"/>
+        </listitem>
+      </varlistentry>
+
       <varlistentry>
         <term><varname>systemd.run=</varname></term>
         <term><varname>systemd.run_success_action=</varname></term>