stub: Read extra kernel command line items from SMBIOS

Let's read more kernel command line arguments from SMBIOS OEM string
io.systemd.stub.kernel-cmdline-extra. This allows adding debug kernel
command line arguments when booting in qemy without having to modify
the UKI.

man/systemd-stub.xml

@@ -379,6 +379,23 @@
     default, this is done for the TPM2 PCR signature and public key files.</para>
   </refsect1>
 
+  <refsect1>
+    <title>SMBIOS Type 11 Strings</title>
+
+    <para><command>systemd-stub</command> can be configured using SMBIOS Type 11 strings. Applicable strings
+    consist of a name, followed by <literal>=</literal>, followed by the value.
+    <command>systemd-stub</command> will search the table for a string with a specific name, and if found,
+    use its value. The following strings are read:</para>
+
+    <variablelist>
+      <varlistentry>
+        <term><varname>io.systemd.stub.kernel-cmdline-extra</varname></term>
+        <listitem><para>If set, the value of this string is added to the list of kernel command line
+        arguments that are passed to the kernel.</para></listitem>
+      </varlistentry>
+    </variablelist>
+  </refsect1>
+
   <refsect1>
     <title>Assembling Kernel Images</title>
 

src/boot/efi/stub.c

@@ -14,6 +14,7 @@
 #include "splash.h"
 #include "tpm-pcr.h"
 #include "util.h"
+#include "vmm.h"
 
 /* magic string to find in the binary image */
 _used_ _section_(".sdmagic") static const char magic[] = "#### LoaderInfo: systemd-stub " GIT_VERSION " ####";
@@ -275,6 +276,12 @@ static EFI_STATUS run(EFI_HANDLE image) {
                 mangle_stub_cmdline(cmdline);
         }
 
+        const char *extra = smbios_find_oem_string("io.systemd.stub.kernel-cmdline-extra");
+        if (extra) {
+                _cleanup_free_ char16_t *tmp = TAKE_PTR(cmdline), *extra16 = xstr8_to_16(extra);
+                cmdline = xasprintf("%ls %ls", tmp, extra16);
+        }
+
         export_variables(loaded_image);
 
         if (pack_cpio(loaded_image,