v2.75.11: Add Ethernet support

The ethernet support allows to make the network/internet connection
via LAN cable instead of WiFi. LAN is preferred, if a LAN cable is
connected and a valid IP configuration via DHCP can be obtained, WiFi
is switched off. If the LAN cable is disconnected, a fall back to
WiFi is done.

Use those defines in platform.ini for PHY settings:
ETHERNET_PHY_POWER: IO pin where the PHY can be switched of/on, can be
                    -1 if not used.
ETHERNET_CLK_MODE: PHY clock mode, see eth_clock_mode_t in ETH.h
ETHERNET_PHY_MDC: PHY MDC pin
ETHERNET_PHY_MDIO: PHY MDIO pin
ETHERNET_PHY_TYPE: PHY type, see eth_phy_type_t in ETH.h

Limitations:
- only DHCP is supported, no static IP configuration for LAN so far.
- If GPIO0 is used for one of the ETHERNET_CLK_MODE modes, then GPIO0
  cannot be used to clear the WiFi configuration.

Signed-off-by: Frank Kunz <mailinglists@kunz-im-inter.net>