Make 'fill' optional with 'split' option enabled

[StefanoPietrosanti]

When generating a .bin boot image with the 'split' option enabled, it would be useful to allow the option to disable the 'fill' option. This allows the generation of smaller files and prevent erasing memory areas which are not to be erased.

This is useful if partitions are to be flashed separately (both in time and memory space) from each others or from the boot code / headers.

Currently the implementation ignores the "doFill" argument: https://github.com/Xilinx/bootgen/blame/1795f18361caf1536d76ffe40ae3cb66e212b020/binfile.cpp#L87

Implementation wise, this would involve a new command line option (e.g. -no-fill) which sets the doFill property https://github.com/Xilinx/bootgen/blob/master/options.h#L267 , maybe checking that the 'split' option is set.