atsamd: Use enumerations for buses and reserve pins

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

src/atsamd/usbserial.c

@@ -10,6 +10,7 @@
 #include "board/irq.h" // irq_disable
 #include "board/usb_cdc.h" // usb_notify_ep0
 #include "board/usb_cdc_ep.h" // USB_CDC_EP_BULK_IN
+#include "command.h" // DECL_CONSTANT_STR
 #include "internal.h" // enable_pclock
 #include "sched.h" // DECL_INIT
 
@@ -184,6 +185,8 @@ usb_request_bootloader(void)
     NVIC_SystemReset();
 }
 
+DECL_CONSTANT_STR("RESERVE_PINS_USB", "PA24,PA25");
+
 void
 usbserial_init(void)
 {