cfi: support for 16-bit flash with reversed endianness

cfi: support for 16-bit flash with reversed endianness

This is for targets where flash controller has reverse endianness
compared to target.  For these, the 'bus_swap' parameter can be given to the
CFI driver, which will cause command CFI commands to be written with
bytes swapped.  This is only for x16 CFI flash.

Change-Id: I698b768e92e65d160232e90b0e81a824e3c81a46
Signed-off-by: Esben Haabendal <esben@haabendal.dk>
Reviewed-on: http://openocd.zylin.com/3041
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>