build: Use compile_time_request system for init, tasks, and shutdown

Avoid using linker magic to define the init, task, and shutdown
functions.  Instead, use the compile_time_request system.  This
simplifies the build and produces more efficient code.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/avr/usbserial.c

@@ -14,7 +14,7 @@ static char receive_buf[USBSERIAL_BUFFER_SIZE];
 static uint8_t receive_pos;
 static char transmit_buf[USBSERIAL_BUFFER_SIZE];
 
-static void
+void
 usbserial_init(void)
 {
     usb_init();