This adds support for slave serial programming, in addition to the
previously supported slave SelectMAP mode. There are two ways that this
can be used:

-Using the clk and wdata callbacks in order to write image data one bit
at a time using pure bit-banging. This works, but is rather painfully
slow with typical image sizes.

-By specifying the wbulkdata callback instead, the image loading process
can be offloaded to SPI hardware. In this mode the clk and wdata
callbacks do not need to be specified. This allows the image to be
loaded much faster, taking only a few seconds with even relatively large
images.

Slave serial programming has been tested on the Kintex-7 series of
FPGAs.

Signed-off-by: Robert Hancock <hancock@sedsystems.ca>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>