AVR8: Add option to keep 3.3V regulator enabled

The documentation contains [sample code on how to restart into the bootloader](http://www.fourwalledcubicle.com/files/LUFA/Doc/130901/html/_page__software_bootloader_start.html). In the process of preparing for the reset, USB is disabled using “USB_Disable()”. For hardware making use of the AVR8's internal 3.3V regulator that call would also disable the regulator, resetting the processor immediately rather than setting the boot key and letting the watchdog reset the processor.

This patch adds a new flag to be given to “USB_Init()” or to be defined in “USE_STATIC_OPTIONS” telling “USB_Disable()” to keep the regulator enabled.

On November 1st, 2013 this issue was already [mentioned on the mailing list](https://groups.google.com/d/msg/lufa-support/uwrFpRQpJzU/e9I6UK5jMJQJ), but no fix came from that discussion.