AVR8: Add option to keep 3.3V regulator enabled

The documentation contains example code[1] on how to restart into the
bootloader. 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[2], but no fix came from that discussion.

[1]
<http://www.fourwalledcubicle.com/files/LUFA/Doc/130901/
html/_page__software_bootloader_start.html>

[2]
<https://groups.google.com/d/msg/lufa-support/uwrFpRQpJzU/e9I6UK5jMJQJ>

Signed-off-by: Michael Hanselmann <public@hansmi.ch>