add primitive esp8266 counter

include/arch/esp8266/driver/counter.h

+#ifndef COUNTER_H
+#define COUNTER_H
+
+extern "C" {
+#include "osapi.h"
+#include "user_interface.h"
+}
+#include "c_types.h"
+
+class Counter {
+	private:
+		Counter(const Counter &copy);
+		uint32_t start_cycles;
+
+	public:
+		Counter() : start_cycles(0) {}
+		uint32_t overflowed;
+
+		inline void start() {
+			asm volatile ("esync; rsr %0,ccount":"=a" (start_cycles));
+		}
+
+		inline uint32_t stop() {
+			uint32_t stop_cycles;
+			asm volatile ("esync; rsr %0,ccount":"=a" (stop_cycles));
+			if (stop_cycles > start_cycles) {
+				return stop_cycles - start_cycles;
+			} else {
+				return 0;
+			}
+		}
+};
+
+extern Counter counter;
+
+#endif

src/arch/esp8266/driver/counter.cc

+#include "driver/counter.h"
+
+Counter counter;