core: introduce ConditionSecurity=measured-os

So far we always conditioned our TPM magic on the UKI having detected
TPM support in the firmware. This is a bit limiting when we want to
support a software TPM that is not visible to the firmware. Hence let's
split this up, and add a separate control that can be set via the kernel
command line.  However, as before, let's by default inherit the firmare
TPM discovery state into it, to retain the current behaviour unless
overriden.

With this in place, boot with "systemd.tpm2_measured_os=1
systemd.tpm2_software_fallback=1" on the kernel cmdline to get the swtpm
fallback and then a measured OS based on it.

man/systemd.unit.xml

@@ -1605,6 +1605,10 @@
                   <entry>measured-uki</entry>
                   <entry>Unified Kernel Image with PCR 11 Measurements, as per <citerefentry><refentrytitle>systemd-stub</refentrytitle><manvolnum>7</manvolnum></citerefentry>. <xi:include href="version-info.xml" xpointer="v255"/></entry>
                 </row>
+                <row>
+                  <entry>measured-os</entry>
+                  <entry>OS PCR measurements enabled. This is typically equivalent to <varname>measured-uki</varname>, however may also be set explicitly via the <varname>systemd.tpm2_measured_os=</varname> kernel command line switch, see <citerefentry><refentrytitle>kernel-command-line</refentrytitle><manvolnum>7</manvolnum></citerefentry> for details. The various system services doing boot and runtime measurements are conditioned on this flag. <xi:include href="version-info.xml" xpointer="v261"/></entry>
+                </row>
               </tbody>
             </tgroup>
           </table>