* new ESPNOWRadio driver

* refactored the examples/*/main.cpp modules, moving radio specifics to variants/*/target modules
* new Generic_ESPNOW_* target envs

variants/generic_espnow/target.h

@@ -0,0 +1,13 @@
+#pragma once
+
+#include <helpers/ESP32Board.h>
+#include <helpers/esp32/ESPNOWRadio.h>
+
+extern ESP32Board board;
+extern ESPNOWRadio radio_driver;
+
+bool radio_init();
+uint32_t radio_get_rng_seed();
+void radio_set_params(float freq, float bw, uint8_t sf, uint8_t cr);
+void radio_set_tx_power(uint8_t dbm);
+mesh::LocalIdentity radio_new_identity();