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.
To test compilation of optional ESP32 modules, these modules are enabled for the `esp32-wrover-kit` borad when compiled in CI. The use of some of these optional modules depend on the use of other modules.