USBAPI Supported Endpoint size

In USBAPI.h there is this comment
```
// This definitions is usefull if you want to reduce the EP_SIZE to 16
// at the moment only 64 and 16 as EP_SIZE for all EPs are supported except the control endpoint
#ifndef USB_EP_SIZE
#define USB_EP_SIZE 64
#endif
```

What needs to be done to have an EP_SIZE of 8 supported?
I'm currently trying to find the isse why the bootkeyboard from hid-project is not working in all cases and during comparing with other devices the only difference i found was that keyboards which support boot protocol use an endpoint size / packet size of 8.

Start in USBAPI.h and trace the existing USB_EP_SIZE handling for the supported 16- and 64-byte endpoint sizes, excluding the control endpoint. Identify every assumption that prevents a size of 8, then verify that boot-protocol keyboard endpoints work with USB_EP_SIZE set to 8 without changing control-endpoint behavior.

Written by the indexing model from the issue text.