circbuf driver was converted to platform driver, some unused lines of
code were commented out to delete them in the future. Currently the
driver is built in into kernel as I could not easily split it to
separate external modules due to various dependencies and issues during
linking stage.

This involves some modifications to meta-elphel layer.

dma_sync_sg_for_[cpu,device] were added. Driver crashes on reading large
block of data.