net: introduce alternative implementation as net/lwip/

Prepare the introduction of the lwIP (lightweight IP) TCP/IP stack by
adding a new net/lwip/ directory and the NET_LWIP symbol. Network
support is either NO_NET, NET (legacy stack) or NET_LWIP. Subsequent
commits will introduce the lwIP code, re-work the NETDEVICE integration
and port some of the NET commands and features to lwIP.

SPL_NET cannot be enabled when NET_LWIP=y. SPL_NET pulls some symbols
that are part of NET (such as arp_init(), arp_timeout_check(),
arp_receive(), net_arp_wait_packet_ip()). lwIP support in SPL may be
added later.

Similarly, DFU_TFTP and FASTBOOT are not compatible with NET_LWIP
because of dependencies on net_loop(), tftp_timeout_ms,
tftp_timeout_count_max and other NET things. Let's add a dependency on
!NET_LWIP for now.

SANDBOX can select NET_LWIP but doing so will currently disable the eth
dm tests as well as the wget tests which have strong dependencies on the
NET code.

Other adjustments to Kconfig files are made to fix "unmet direct
dependencies detected" for USB_FUNCTION_SDP and CMD_FASTBOOT when
the default networking stack is set to NET_LWIP ("default NET_LWIP"
instead of "default NET" in Kconfig).

The networking stack is now a choice between NO_NET,
NET and NET_LWIP. Therefore '# CONFIG_NET is not set' should be
'CONFIG_NO_NET=y'. Adjust the defconfigs accordingly.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>