PHYLIB depends on NETDEVICES, so to avoid unmet dependencies warning
from Kconfig it needs to be selected conditionally.

Also add checks if PHYLIB is built-in to avoid undefined references to
PHYLIB's symbols.
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Scott Wood <oss@buserror.net>