init: Initialize serial subsystem before init_earlyinitcalls()

Otherwise, the serial sybsystem isn't initialized before the serial
drivers attempt to register themselves.

drivers/serial.c

@@ -33,8 +33,12 @@ struct serial_dev *serial_first_device() {
 	return first_serial_dev;
 }
 
+/*
+ * Initialize the serial sybsystem.
+ * Note: this is called directly from the kernel initialization as opposed to
+ * being an initcall because it has to be called before the serial drivers get
+ * initialized in earlyinitcalls.
+ */
 void serial_init() {
 	first_serial_dev = 0;
 }
-
-driversubsys_initcall(serial_init);

kernel/start_kernel.c

@@ -64,6 +64,7 @@ void serial_console_init() {
 		print_init(sdev->putc);
 }
 
+void serial_init();
 void kmalloc_init();
 
 int main(void) {
@@ -73,8 +74,13 @@ int main(void) {
 	//setup MMU
 	mmu_reinit();
 
+	/* Initialize the serial subsystem before
+	 * init_earlyinitcalls(), because console drivers get
+	 * initialized here so as to have an output console as
+	 * early as possible, and we don't want those
+	 * initializations to fail. */
+	serial_init();
 	init_earlyinitcalls();
-
 	serial_console_init();
 
 	//setup memory