support gdbstub use serial.

we can use uart to debug nuttx,like debugger:

1. read/write memory
2. Use watchpoint,breakpoint,single step.
    use up_debugpoint api

3. Ctrl+c to stop, continue, or single step.
    hold uart send and receive

4. register a panic event, when crash or assert/panic, we use uart to
   debug.

Signed-off-by: anjiahao <anjiahao@xiaomi.com>

drivers/serial/Kconfig

@@ -41,6 +41,18 @@ config SERIAL_CONSOLE
 	bool
 	default n
 
+config SERIAL_GDBSTUB
+	bool "GDBSTUB Serial support"
+	depends on ARCH_HAVE_DEBUG
+	default n
+
+config SERIAL_GDBSTUB_PATH
+	string "GDBSTUB Serial path"
+	depends on SERIAL_GDBSTUB
+	default "/dev/ttyS1"
+	---help---
+		The path to the serial device that will be used for GDB stub.
+
 menuconfig UART_PL011
 	bool "PL011 Chip support"
 	default n