add interrupt latency printout command and mean/variance to interval performance counter

src/systemcmds/perf/perf.c

@@ -68,8 +68,12 @@ int perf_main(int argc, char *argv[])
 		if (strcmp(argv[1], "reset") == 0) {
 			perf_reset_all();
 			return 0;
+		} else if (strcmp(argv[1], "latency") == 0) {
+			perf_print_latency(0 /* stdout */);
+			fflush(stdout);
+			return 0;
 		}
-		printf("Usage: perf <reset>\n");
+		printf("Usage: perf [reset | latency]\n");
 		return -1;
 	}