busybox: Install udhcpc script only if enabled

It doesn't make sense to install such a scripts if UDHCPC was not
selected.
Also remove the entry from BUSYBOX_PERMISSIONS as the install command
does the same job.

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>