svf: try to reallocate buffers if they're not enough

svf: try to reallocate buffers if they're not enough

On larger FPGAs the default buffer size is not nearly enough;
automatically reallocate it as needed.

Change-Id: Ic8071e8cceddeef897ce76996c029250fd22662b
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2099
Tested-by: jenkins
Reviewed-by: Kenneth Lorthioir <ibelimb@gmail.com>
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>