kernel: implement support for magenta_debug over serial

arch/x86_64/init.c

@@ -7,6 +7,7 @@
 #include <kernel/clock.h>
 #include <kernel/console.h>
 #include <kernel/cpu.h>
+#include <kernel/debug.h>
 #include <kernel/init.h>
 #include <kernel/libc/stdio.h>
 #include <kernel/machine/cpu.h>
@@ -19,6 +20,7 @@
 #include <kernel/types.h>
 #include <kernel/util.h>
 #include <kernel/vm.h>
+#include <magenta/debug/packet.h>
 
 #define HARDWARE_RNG
 
@@ -69,7 +71,8 @@ static void early_vm_init(uintptr_t reserve_end)
 void early_console_init(void)
 {
 	const char *dest = arg_value("kernel.early-console");
-	if (!dest) {
+	bool debug = arg_is_set("kernel.enable-debugger");
+	if (!dest || debug) {
 		dest = "tty0";
 	}
 
@@ -186,5 +189,10 @@ int ml_init(uintptr_t arg)
 	pit_start(500);
 	ml_int_enable();
 
+	if (arg_is_set("kernel.enable-debugger")) {
+		debug_await();
+		debug_create_task();
+	}
+
 	return 0;
 }