Add platform dfu to restart the STM32 into USB DFU

[evoggy]

Adds cfcli platform dfu, which restarts the STM32 into its ROM bootloader so it can be flashed
over USB DFU, without holding the power button.

Depends on bitcraze/crazyflie2-nrf-firmware#126, which adds the nRF51 command this sends.

How it works

platform dfu sends 0xFF 0xFE 0x07 to the nRF51, the same way sleep, wakeup and
power-off send their commands. The nRF51 then power-cycles the STM32 with BOOT0 high, and cfcli
waits up to 5 s for it to show up on USB as 0483:df11:

$ cfcli platform dfu
STM32 is in USB DFU mode (0483:df11)
When done, restart it into its firmware with: cfcli platform reboot

• Radio only. The STM32 firmware does not pass these commands on to the nRF51 from USB, so a
  usb:// URI is refused with an error instead of silently doing nothing.
• When no DFU device appears, the command fails. Either the Crazyflie is not connected over
  USB, or its nRF51 firmware does not have the command yet (older firmware ignores it). The
  message points at cfcli platform reboot in case the STM32 did restart.
• docs/platform.md has a new section for the command.

Testing

On a Crazyflie 2.1 Brushless running nRF51 firmware with #126, connected over the radio and USB:

1. cfcli platform dfu succeeded, and the DFU device was listed by dfu-util -l with all four alt
   settings.
2. The STM32 bootloader was flashed with dfu-util and read back to match.
3. cfcli platform reboot brought the Crazyflie back to its firmware.

cargo test passes.