USB connected w/o battery does not go into bootloader

[knmcguire]

This is related to this issue in the NRF firmware repo: bitcraze/crazyflie2-nrf-firmware#35

The NRF builder would need to be rebuild with make clean all USES_RFX2411N=1 USES_TI_CHARGER=1 but these should be standard.

[knmcguire]

This was fixed after the first few batches of CF 2.1s, so this can be closed.

This should be documented in a troubleshooting guide thought for the first batches