rp2040: Move chipid reading to bootrom.c

Rewrite chipid.c so that it contains just the USB and canbus id
manipulation code.  Move the low-level chipid reading to bootrom.c.

Also, introduce a new bootrom_reboot_usb_bootloader() function in
bootrom.c so that the main.c code does not need to know the specifics
of rebooting into the bootrom.

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

src/rp2040/main.c

@@ -51,7 +51,7 @@ bootloader_request(void)
     try_request_canboot();
     // Use the bootrom-provided code to reset into BOOTSEL mode
     if (CONFIG_MACH_RP2040)
-        reset_to_usb_boot(0, 0);
+        bootrom_reboot_usb_bootloader();
 }