[monojob] Allow monojob to be completely silent

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/monojob.c

 /**
  * Wait for single foreground job to complete
  *
- * @v string		Job description to display
+ * @v string		Job description to display, or NULL to be silent
  * @ret rc		Job final status code
  */
 int monojob_wait ( const char *string ) {
 	unsigned int percentage;
 	int shown_percentage = 0;
 
-	printf ( "%s...", string );
+	if ( string )
+		printf ( "%s...", string );
 	monojob_rc = -EINPROGRESS;
 	last_progress = currticks();
 	while ( monojob_rc == -EINPROGRESS ) {
 			}
 		}
 		elapsed = ( currticks() - last_progress );
-		if ( elapsed >= TICKS_PER_SEC ) {
+		if ( string && ( elapsed >= TICKS_PER_SEC ) ) {
 			if ( shown_percentage )
 				printf ( "\b\b\b\b    \b\b\b\b" );
 			job_progress ( &monojob, &progress );
 	if ( shown_percentage )
 		printf ( "\b\b\b\b    \b\b\b\b" );
 
-	if ( rc ) {
-		printf ( " %s\n", strerror ( rc ) );
-	} else {
-		printf ( " ok\n" );
+	if ( string ) {
+		if ( rc ) {
+			printf ( " %s\n", strerror ( rc ) );
+		} else {
+			printf ( " ok\n" );
+		}
 	}
+
 	return rc;
 }