instruments: Add screencap

Add an instrument which wraps the devlib screencapture poller, which
allows us to capture a screenshot from the device every `period`. The
`period` is configurable based on the user requirements.

All captured screenshots are named according to the timestamp on the
device, and are placed into an output directory (screen-capture)
within the job directory.

wa/instruments/screencap.py

@@ -0,0 +1,57 @@
+#    Copyright 2018 ARM Limited
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+import os
+
+from devlib.trace.screencapture import ScreenCaptureCollector
+
+from wa import Instrument, Parameter
+
+
+class ScreenCaptureInstrument(Instrument):
+
+    name = 'screen_capture'
+    description = """
+    A simple instrument which captures the screen on the target devices with a user-specified period.
+
+    Please note that if a too short period is specified, then this
+    instrument will capture the screen as fast as possible, rather
+    than at the specified periodicity.
+    """
+
+    parameters = [
+        Parameter('period', kind=int, default=10,
+                  description="""
+                  Period (in seconds) at which to capture the screen on the target.
+                  """),
+    ]
+
+    def __init__(self, target, **kwargs):
+        super(ScreenCaptureInstrument, self).__init__(target, **kwargs)
+        self.collector = None
+
+    def setup(self, context):
+        # We need to create a directory for the captured screenshots
+        output_path = os.path.join(context.output_directory, "screen-capture")
+        os.mkdir(output_path)
+        self.collector = ScreenCaptureCollector(self.target,
+                                                output_path,
+                                                self.period)
+
+    def start(self, context):
+        self.collector.start()
+
+    def stop(self, context):
+        self.collector.stop()