gdbstub: make immediate GDB stub entry optional

Add a Kconfig option to be able to postpone entering the GDB stub
until an external trigger.

Signed-off-by: Noah Klayman <noah.klayman@intel.com>
Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>

subsys/debug/gdbstub/Kconfig

@@ -42,4 +42,11 @@ config GDBSTUB_TRACE
 	  Enable extra debug logging for the GDB backend, including
 	  breakpoint interrupts and remote commands it receives.
 
+config GDBSTUB_ENTER_IMMEDIATELY
+	bool "Stop execution and enter the GDB stub at boot"
+	default y
+	help
+	  Functions like QEMU's -S. If enabled, boot will halt before user code
+	  starts execution and GDB stub will wait for connection.
+
 endif # GDBSTUB

subsys/debug/gdbstub/gdbstub.c

@@ -891,6 +891,7 @@ int gdb_init(void)
 	return 0;
 }
 
+#ifdef CONFIG_GDBSTUB_ENTER_IMMEDIATELY
 #ifdef CONFIG_XTENSA
 /*
  * Interrupt stacks are being setup during init and are not
@@ -904,3 +905,4 @@ SYS_INIT(gdb_init, POST_KERNEL, CONFIG_KERNEL_INIT_PRIORITY_DEFAULT);
 #else
 SYS_INIT(gdb_init, PRE_KERNEL_2, CONFIG_KERNEL_INIT_PRIORITY_DEFAULT);
 #endif
+#endif