condition: introduce systemd.condition-first-boot= kernel command line switch

Much like systemd.condition-needs-update= this new switch allows
overriding of a unit file condition, but this time its
ConditionFirstBoot=.

Usecase is also primarily debugging, but could be useful for other
schemes too.

src/shared/condition.c

@@ -627,11 +627,18 @@ static int condition_test_needs_update(Condition *c, char **env) {
 
 static int condition_test_first_boot(Condition *c, char **env) {
         int r, q;
+        bool b;
 
         assert(c);
         assert(c->parameter);
         assert(c->type == CONDITION_FIRST_BOOT);
 
+        r = proc_cmdline_get_bool("systemd.condition-first-boot", &b);
+        if (r < 0)
+                log_debug_errno(r, "Failed to parse systemd.condition-first-boot= kernel command line argument, ignoring: %m");
+        if (r > 0)
+                return b == !!r;
+
         r = parse_boolean(c->parameter);
         if (r < 0)
                 return r;