rcld: Use static empty buffer for zeroing cl buffers

Signed-off-by: Pawel Dziepak <pdziepak@quarnos.org>
rcld: Make cl_write_buffer asynchronous
 * clEnqueueWriteBuffer is made non blocking. The buffer data is written from
   is later released by OpenCL completion event callback
 * command queue is synchronized before reading from the device and executing
   a kernel, no need to synchronize after executing a kernel

Signed-off-by: Pawel Dziepak <pdziepak@quarnos.org>