The ESP32 Ethernet Kit has only a single GPIO pin exposed to SAUL via
`saul_gpio`, but that GPIO doubles as PHY clock input when `esp32_eth`
is used. Hence, the `saul_gpio` dep should be optional and only kick
in when `esp32_eth` is not used.
This removes the configuration parameters for mrf24j40 and enc28j60
drivers, which were present in the boards's Makefile.dep. There is
already documentation regarding how to correctly configure these devices
for the board.
