Using `gnrc_border_router` with `uhcp` is quite noisy.
uhcpc will regularly refresh the prefix and print a bunch of status messages.
Allow the user to tone it down by setting a higher `LOG_LEVEL`.
For this, convert calls to `printf()` and `puts()` to `LOG_xxx()`.
This requires a dummy header for `uhcpd`.
Adds '--bind-to-device' option to binds uhcpd listening socket to a device.
It allows running multiple uhcpd and so multiple border router on the same
computer.
This is used for IoT-LAB.
This option requires 'root' permissions as I currently didn't find an
alternative.
