PCA9685 PWM driver support (#261)

* 1st stage of PCA9685 PWM driver support
* Servo output to PCA9685 in NAZE target.
* FEATURE_PWM_SERVO_DRIVER introduced
* task for PWM driver
* abstraction layer for pwm drivers
* feature enabled only on target with >128 flash

src/main/io/pwmdriver_i2c.c

@@ -0,0 +1,57 @@
+#include <stdbool.h>
+#include <stdint.h>
+#include "drivers/io_pca9685.h"
+
+#include "config/config.h"
+#include "fc/runtime_config.h"
+#include "config/feature.h"
+
+#define PWM_DRIVER_IMPLEMENTATION_COUNT 1
+#define PWM_DRIVER_MAX_CYCLE 4
+
+static bool driverEnabled = false;
+static uint8_t driverImplementationIndex = 0;
+
+typedef struct {
+    bool (*initFunction)(void);
+    void (*writeFunction)(uint8_t servoIndex, uint16_t off);
+    void (*setFrequencyFunction)(float freq);
+    void (*syncFunction)(uint8_t cycleIndex);
+} pwmDriverDriver_t;
+
+pwmDriverDriver_t pwmDrivers[PWM_DRIVER_IMPLEMENTATION_COUNT] = {
+    [0] = {
+        .initFunction = pca9685Initialize,
+        .writeFunction = pca9685setServoPulse,
+        .setFrequencyFunction = pca9685setPWMFreq,
+        .syncFunction = pca9685sync
+    }
+};
+
+bool isPwmDriverEnabled() {
+    return driverEnabled;
+}
+
+void pwmDriverSetPulse(uint8_t servoIndex, uint16_t length) {
+    (pwmDrivers[driverImplementationIndex].writeFunction)(servoIndex, length);
+}
+
+void pwmDriverInitialize(void) {
+    driverEnabled = (pwmDrivers[driverImplementationIndex].initFunction)();
+
+    if (!driverEnabled) {
+        featureClear(FEATURE_PWM_SERVO_DRIVER);
+    }
+
+}
+
+void pwmDriverSync(void) {
+    static uint8_t cycle = 0;
+
+    (pwmDrivers[driverImplementationIndex].syncFunction)(cycle);
+
+    cycle++;
+    if (cycle == PWM_DRIVER_MAX_CYCLE) {
+        cycle = 0;
+    }
+}