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.

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,