How to add variant configurations after initializing the USB stack

Hi all,
We're making the 3-party boards add-on package for Arduino 1.6.4. Our board need to be re-configured must after the USB stack being initialized, or our configuration will be overrided by the USB stack. Just like this:

``` C++
int main(void)
{
init();

initVariant();

#if defined(USBCON)
USBDevice.attach();
#endif

#if defined(BLEND_MICRO_8MHZ)
// As the F_CPU = 8000000UL, the USB core make the PLLCSR = 0x02
// But the external xtal is 16000000Hz, so correct it here.
PLLCSR |= 0x10; // Need 16 MHz xtal
while (!(PLLCSR & (1<