sysext: provide systemd-{sysext,confext}-sysroot.service services (#41161)

This should pretty much close #38985

The new services are used to activate system and configuration
extensions for the main system from the initrd, this allows to overcome
the limitation that sysext/confext cannot be used to update the
resources which are required in the earliest boot of the system (before
systemd-sysext/systemd-confext start).

To make it possible to disable sysext/confext merging logic,
`systemd.sysext=0`, `systemd.confext=0`, `rd.systemd.sysext=0`,
`rd.systemd.confext=0` kernel cmdline options are introduced.

Closes #38985

src/sysext/sysext.c

@@ -51,6 +51,7 @@
 #include "path-util.h"
 #include "pidref.h"
 #include "pretty-print.h"
+#include "proc-cmdline.h"
 #include "process-util.h"
 #include "rm-rf.h"
 #include "runtime-scope.h"
@@ -3038,6 +3039,22 @@ static int run(int argc, char *argv[]) {
         if (r <= 0)
                 return r;
 
+        /* PROC_CMDLINE_STRIP_RD_PREFIX cannot be used here as we need to be able to distinguish between
+         * rd.systemd.{sysext,confext} and systemd.{sysext,confext} in the initrd where they are both used
+         * and have different meaning. */
+        const char *string_class = image_class_to_string(arg_image_class);
+        const char *cmdline_opt = strjoina(in_initrd() && !arg_root ? "rd." : "", "systemd.", string_class);
+
+        bool enabled;
+        r = proc_cmdline_get_bool(cmdline_opt, PROC_CMDLINE_TRUE_WHEN_MISSING, &enabled);
+        if (r < 0)
+                log_debug_errno(r, "Failed to check '%s=' kernel command line option, proceeding: %m", cmdline_opt);
+        else if (!enabled && invoked_by_systemd()) {
+                /* Kernel command line option should not affect manual invocation. */
+                log_notice("Disabled by the kernel command line option '%s=', skipping execution.", cmdline_opt);
+                return 0;
+        }
+
         /* Parse configuration file after argv because it needs --root=.
          * The config entries will not overwrite values set already by
          * env/argv because we track initialization. */