[Bootloader] Enable USB string descriptors

[sandeepmistry]

Add support for sending USB string descriptors for manufacturer and product in the boot loader.

The following error is no longer present in OS X 10.11.3 kernel logs, after these boot loader changes are applied.

2016-01-25 4:33:08.000 PM kernel[0]: 129020.020709 IOUSBHostDevice@14200000: IOUSBHostDevice::getDescriptorGated: type 0x03 index 0 length 2 completed with 0xe0005000 and bytesTransferred 0
2016-01-25 4:33:08.000 PM kernel[0]: 129020.022294 IOUSBHostDevice@14200000: IOUSBHostDevice::getDescriptorGated: type 0x03 index 0 length 2 completed with 0xe0005000 and bytesTransferred 0

Tested with OS X 10.11.3, and Linux (Rasbian Jessie).

cc/ @aethaniel

[ArduinoBot]

✅ Build completed.

⬇️ Build URL: http://downloads.arduino.cc/PR/samd/package_samd-b49_index.json

ℹ️ To test this build:

1. Open the Preferences of the Arduino IDE.
2. Add the Build URL above in the Additional Boards Manager URLs field, and click OK.
3. Open the Boards Manager (menu Tools->Board->Board Manager...)
4. Install Arduino SAMD core - Pull Request [Bootloader] Enable USB string descriptors #104
5. Select one of the boards under SAMD Pull Request [Bootloader] Enable USB string descriptors #104 in Tools->Board menu
6. Compile/Upload as usual

[cmaglie]

Superseded by #165