udev: don't hide if busybox is enabled

udev is hidden by default (because of BR2_PACKAGE_BUSYBOX_SHOW_OTHERS),
but other packages (hal + usbmount) tries to select it, which gives
warnings if it isn't visible.

To fix this, simply make it always visible. To call mdev an equivalent
of udev is also stretching the truth anyway.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>