drivers/serial/Kconfig: Make feature dependent on CONFIG_BUILD_FLAT.

drivers/serial/Kconfig

@@ -23,7 +23,7 @@ config SERIAL_SIGKILL_CHAR
 	int "Serial parse SIGKILL characters"
 	default 3 if SERIAL_CONSOLE
 	default 4 if !SERIAL_CONSOLE
-	depends on SIG_SIGKILL
+	depends on SIG_SIGKILL && BUILD_FLAT
 	---help---
 		Use ASCII 3 (Ctrl-c) or 4 (ctrl-d) inputs to determine whether to
 		send a SIGKILL event.  Other charcters may also be selected.

drivers/serial/serial.c

@@ -1371,6 +1371,8 @@ static int uart_ioctl(FAR struct file *filep, int cmd, unsigned long arg)
 #endif
 
 #ifdef CONFIG_SERIAL_SIGKILL_CHAR
+          /* Make the given terminal the controlling terminal of the calling process */
+
           case TIOCSCTTY:
             {
               /* REVISIT:  This only applies to console devices (TTYs).  In