* 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.cpp

@@ -0,0 +1,29 @@
+#include <Arduino.h>
+#include "target.h"
+#include <helpers/ArduinoHelpers.h>
+
+ESP32Board board;
+
+ESPNOWRadio radio_driver;
+
+bool radio_init() {
+  // NOTE: radio_driver.begin() is called by Dispatcher::begin(), so not needed here
+  return true;  // success
+}
+
+uint32_t radio_get_rng_seed() {
+  return millis() + radio_driver.intID();  // TODO: where to get some entropy?
+}
+
+void radio_set_params(float freq, float bw, uint8_t sf, uint8_t cr) {
+  // no-op
+}
+
+void radio_set_tx_power(uint8_t dbm) {
+  radio_driver.setTxPower(dbm);
+}
+
+mesh::LocalIdentity radio_new_identity() {
+  StdRNG  rng;  // TODO: need stronger True-RNG here
+  return mesh::LocalIdentity(&rng);  // create new random identity
+}