platform: Add PWM property with variable cycle (#97)

It was tested on Intel Edison PWM0 (50 Hz on buzzer)

Note that GPIO12 must be manually configured first, using:

```sh
gpio=12
echo $gpio | sudo tee -a /sys/class/gpio/export
echo out | sudo tee /sys/class/gpio/gpio$gpio/direction
echo mode1 | sudo tee /sys/kernel/debug/gpio_debug/gpio${gpio}/current_pinmux
```

More/Better gpio drivers maybe added later,
for the record 'pwm' one was prefered here
as base for webthing-iotjs (API to be aligned with IoT.js),
but pwm module is lacking muxing support.

It can be used to control servo motors, for more check:

https://social.samsunginter.net/@rzr/102343690505588635#

Relate-to: djdeath/pwm#2
Change-Id: Ia54efc1d3ef232a831e46f324b9f582a772fceec
Signed-off-by: Philippe Coval <p.coval@samsung.com>

example/platform/pwm/pwm-property.js

@@ -0,0 +1,114 @@
+// -*- mode: js; js-indent-level:2;  -*-
+// SPDX-License-Identifier: MPL-2.0
+
+/**
+ *
+ * Copyright 2018-present Samsung Electronics France SAS, and other contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/
+ */
+const console = require('console');
+
+// Disable logs here by editing to '!console.log'
+const log = console.log || function() {};
+const verbose = !console.log || function() {};
+
+const {
+  Property,
+  Value,
+} = require('webthing');
+
+const pwm = require('pwm');
+
+class PwmOutProperty extends Property {
+  constructor(thing, name, value, metadata, config) {
+    if (typeof config === 'undefined') {
+      config = {};
+    }
+    super(thing, name, new Value(Number(value)),
+          {
+            '@type': 'LevelProperty',
+            title: (metadata && metadata.title) || `PWM: ${name} (dutyCycle)`,
+            type: 'integer',
+            minimum: config.minimum || 0,
+            maximum: config.maximum || 100,
+            readOnly: false,
+            unit: 'percent',
+            description:
+            (metadata && metadata.description) ||
+              (`PWM DutyCycle`),
+          });
+    const self = this;
+    this.config = config;
+    if (typeof this.config.pwm == 'undefined') {
+      this.config.pwm = {};
+    }
+    if (typeof this.config.pwm.pin == 'undefined') {
+      this.config.pwm.pin = 0;
+    }
+
+    if (typeof this.config.pwm.chip == 'undefined') {
+      this.config.pwm.chip = 0;
+    }
+    // secs (eg: 50Hz = 20 ms = 0.02 sec)
+    if (typeof this.config.pwm.period == 'undefined') {
+      this.config.pwm.period = 0.02;
+    }
+    // [0..1]
+    if (typeof this.config.pwm.dutyCycle == 'undefined') {
+      this.config.pwm.dutyCycle = 0.5;
+    }
+    verbose(`log: opening: ${this.getName()}`);
+    this.port = pwm.export(
+      this.config.pwm.chip, this.config.pwm.pin,
+      (err) => {
+        verbose(`log: PWM: ${self.getName()}: open: ${err}`);
+        if (err) {
+          console.error(`error: PWM: ${self.getName()}: open: ${err}`);
+          throw err;
+        }
+        self.port.freq = 1 / self.config.pwm.period;
+        // Linux sysfs uses usecs units
+        self.port.setPeriod(
+          self.config.pwm.period * 1.E9,
+          () => {
+            self.port.setDutyCycle(
+              self.config.pwm.dutyCycle * (self.config.pwm.period * 1.E9),
+              () => {
+                self.port.setEnable(1, () => {
+                  verbose(`log: ${self.getName()}: Enabled`);
+                });
+              });
+          });
+
+        self.value.valueForwarder = (value) => {
+          const usec = Math.floor((self.config.pwm.period * 1.E9) *
+                                  (Number(value) / 100.0));
+          self.port.setDutyCycle(usec, () => {
+            verbose(`log: setDutyCycle: usec=${usec}`);
+          });
+        };
+      });
+  }
+
+  close() {
+    verbose(`log: PWM: ${this.getName()}: close:`);
+    try {
+      this.port && this.port.unexport();
+    } catch (err) {
+      console.error(`error: PWM: ${this.getName()} close:${err}`);
+      return err;
+    }
+    log(`log: PWM: ${this.getName()}: close:`);
+  }
+}
+
+
+module.exports = PwmOutProperty;
+
+
+if (module.parent === null) {
+  new PwmOutProperty;
+}