Hardcoded timeout in USB_Send

Here:
[u8 timeout = 250](https://github.com/arduino/ArduinoCore-avr/blob/bc792a9a716289249fa5f2f67be3ee8ba6507d6c/cores/arduino/USBCore.cpp#L280)

250ms is quite large for my application. Could this be moved to a compile time define?

Perhaps something like this?
```
#ifndef USB_SEND_TIMEOUT
#define USB_SEND_TIMEOUT 250
#endif
```

```diff
- u8 timeout = 250; // 250ms timeout on send? TODO
+ u8 timeout = USB_SEND_TIMEOUT; // timeout on send? TODO
```

Start at cores/arduino/USBCore.cpp around line 280, where USB_Send contains the hardcoded timeout. Review the linked pull request and the proposed compile-time configuration in the issue. Done means the timeout can be configured at compile time while retaining the existing default.

Written by the indexing model from the issue text.