Now the release() function only performs the action that is called for,
i.e. release the endpoint and let it receive data. All the buffers handling
has been inlined in the callers, this also slightly improves performance
because it allows to remove some redundant checks.

The reference to the upper USBDevice class is passed on
the EPHandler constructor.

this is the proper type to encapsulate a byte when data is available
or -1 when there is no data.

Fixed PWM frequency glitch

Port of https://github.com/arduino/Arduino/pull/4718 to SAMD core.

Use TX buffer for Wire slave writes

Enable digitalRead on output pins, only enable pull-up in digitalWrite if pin is not output

Add Serial_::availableForWrite()

Fix #149

Add Uart::availableForWrite()

Correct SerialUSB return value

Port of https://github.com/arduino/Arduino/pull/4996