'Read contents of flash back, return to caller.'

def readback(self, offset, length):

    tf = tempfile.NamedTemporaryFile(delete=False)
    self.tempfiles.append(tf.name)
    tf.close()
    self.run_esptool(('read_flash    %d    %d    %s' % (offset, length, tf.name)))
    with open(tf.name, 'rb') as f:
        rb = f.read()
    self.assertEqual(length, len(rb), ('read_flash    length    %d    offset    0x%x    yielded    %d    bytes!' % (length, offset, len(rb))))
    return rb
