ports: On cold boot, enable USB after boot.py completes.

For mimxrt, nrf, renesas-ra, rp2 and samd ports, this commit implements
similar behaviour to the stm32 port, where USB is only brought up after
boot.py completes execution.

Currently this doesn't add any useful functionality (and may break
workflows that depend on USB-CDC being live in boot.py), however it's a
precondition for more usable workflows with USB devices defined in
Python (allows setting up USB interfaces in boot.py before the device
enumerates for the first time).

This work was funded through GitHub Sponsors.

Signed-off-by: Angus Gratton <angus@redyak.com.au>

ports/samd/main.c

@@ -33,6 +33,7 @@
 #include "shared/runtime/gchelper.h"
 #include "shared/runtime/pyexec.h"
 #include "shared/runtime/softtimer.h"
+#include "shared/tinyusb/mp_usbd.h"
 
 extern uint8_t _sstack, _estack, _sheap, _eheap;
 extern void adc_deinit_all(void);
@@ -56,6 +57,9 @@ void samd_main(void) {
 
         // Execute user scripts.
         int ret = pyexec_file_if_exists("boot.py");
+
+        mp_usbd_init();
+
         if (ret & PYEXEC_FORCED_EXIT) {
             goto soft_reset_exit;
         }

ports/samd/samd_soc.c

@@ -62,8 +62,6 @@ static void usb_init(void) {
     PORT->Group[0].PMUX[12].reg = alt << 4 | alt;
     PORT->Group[0].PINCFG[24].reg = PORT_PINCFG_PMUXEN;
     PORT->Group[0].PINCFG[25].reg = PORT_PINCFG_PMUXEN;
-
-    tusb_init();
 }
 
 // Initialize the µs counter on TC 0/1 or TC4/5