net: config: rely on type rather than number of services configured

So far net config initialization code relied on number of services
needed by application. This was fine as long as all enabled
services (e.g. CONFIG_NET_IPV4=y, CONFIG_NET_IPV6=y) were selected by
project configuration as "needed" (e.g. CONFIG_NET_CONFIG_NEED_IPV4=y,
CONFIG_NET_CONFIG_NEED_IPV6=y). Problem appeared for example when both
IPv4 and IPv6 were enabled (CONFIG_NET_IPV4=y, CONFIG_NET_IPV6=y), but
only IPv6 was marked as "needed" (CONFIG_NET_CONFIG_NEED_IPV6=y). In
such situation number of required services was equal to 1. When IPv4
setup was completed, this resulted in returning from net_config_init()
function. Application code failed, because IPv6 was still not
functional.

Do not rely on number of services anymore, as it is error prone. Use
flags instead to mark which services are ready. Compare those flags with
the flags passed to net_config_init() (selected in project configuration
in most cases) to decide whether network configuration has completed
already or not.

Also introduce services_notify_ready() and services_are_ready() helper
functions to isolate implementation details from the logic.

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>