net: Make NET imply NETDEVICES

Normally, when NET is enabled, CMD_NET will then be enabled and in turn
NETDEVICES will (likely) be enabled via imply. However, if we disable
CMDLINE in a defconfig we now no longer get CMD_NET enabling NETDEVICES
for us. This suggestion (as an imply is) really isn't about the network
commands but network itself and is a legacy of how intertwined
NET/CMD_NET were historically. Move this over to the NET entry instead
where it is a more logical fit.

Reported-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>