Add webUSB hardware "core"

[facchinm]

this allows a seamless integration of webUSB enabled boards, allowing IDE and AVR core update

works with arduino/Arduino#5128 applied

[reillyeon]

Where do you get the WebUSB versions of the bootloader images and how do you get the Arduino IDE to pick up these additional board configurations?

[facchinm]

In this branch there are a couple of experiments on WebUSB capable bootloaders, which should be added to hardware/webusb/avr/bootloaders/ folder once ready.
To get the IDE pick up this "core", simply copy the hardware folder into your sketchbook, restart the IDE and it will appear in the Boards menu