platform: Add PWM support

It was tested on Intel edison on PWM0 but
GPIO12 must be manually configured first

```chip=0
id=0
gpio=12
echo $gpio | sudo tee /sys/class/gpio/export
echo out | sudo tee /sys/class/gpio/gpio$gpio/direction
echo 0 | sudo tee /sys/class/gpio/gpio$gpio/value
sudo cat /sys/kernel/debug/gpio_debug/gpio$gpio/current_pinmux # mode0
echo mode1 | sudo tee /sys/kernel/debug/gpio_debug/gpio${gpio}/current_pinmuxsh
```

More/Better gpio drivers maybe used later,
'pwm' one was used here as base for webthing-iotjs.

Relate-to: #3
Change-Id: I6f587a840e8c429d91ba2491e7cddfdfab66bf4b
Signed-off-by: Philippe Coval <p.coval@samsung.com>

example/platform/board/edison.js

@@ -0,0 +1,47 @@
+// -*- 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 {
+  Thing,
+} = require('webthing');
+
+const PwmProperty = require('../pwm/pwm-property');
+
+class EdisonThing extends Thing {
+  constructor(name, type, description) {
+    super(name || 'Edison',
+          type || [],
+          description || 'A web connected Edison');
+    const self = this;
+    this.pinProperties = [
+      new PwmProperty(this, 'PWM0', 50, {
+        description: 'Analog port of Edison',
+      }),
+    ];
+    this.pinProperties.forEach((property) => {
+      self.addProperty(property);
+    });
+  }
+
+  close() {
+    this.pinProperties.forEach((property) => {
+      property.close && property.close();
+    });
+  }
+}
+
+module.exports = function() {
+  if (!module.exports.instance) {
+    module.exports.instance = new EdisonThing();
+  }
+  return module.exports.instance;
+};

example/platform/package.json

@@ -28,6 +28,7 @@
   },
   "homepage": "https://github.com/rzr/webthing-iotjs",
   "dependencies": {
-    "gpio": "^0.2.10"
+    "gpio": "^0.2.10",
+    "pwm": "0.0.3"
   }
 }

example/platform/pwm/pwm-property.js

@@ -0,0 +1,111 @@
+// -*- 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 (!this.config.pwm) {
+      this.config.pwm = {
+        chip: 0,
+        pin: 0,
+        dutyCycle: 0.5, // secs
+        period: 1,
+      };
+    }
+    if (typeof this.config.pwm.dutyCycle == 'undefined') {
+      this.config.pwm.dutyCycle = 0.5;
+    }
+    verbose(`log: opening: ${this.description}`);
+    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 * 1000 * 1000,
+          () => {
+            self.port.setDutyCycle(
+              self.config.pwm.dutyCycle / 100 * 1000 * 1000,
+              () => {
+                self.port.setEnable(1, () => {
+                  verbose(`log: ${self.getName()}: Enabled`);
+                });
+              });
+          });
+
+        self.value.valueForwarder = function(value) {
+          const usec = Math.floor((self.config.pwm.period * 1000 * 1000) *
+                                (Number(value) / 100.0));
+
+          self.port.setDutyCycle(usec, function() {
+            verbose(`log: setDutyCycle: usec=${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;
+  setInterval(function() {
+    console.log(new Date());
+  }, 10000);
+}