tools: copyfile: use 64k instead of 512 buffer

This is a trivial but significant optimization:
mkimage took >200ms (and 49489 writes (of which 49456 512)),
now it takes  110ms (and   419 writes (of which   386 64k)).

sendfile is much more appropriate for this and is done in one syscall,
but doesn't bring any significant speedups over 64k r/w
at the 13M size ranges, so there's no need to introduce
	#if __linux__
	while((size = sendfile(fd_dst, fd_src, NULL, 128 * 1024 * 1024)) > 0)
		;
	if(size != -1) {
		ret = 0;
		goto out;
	}
	#endif

Also extract the buffer size to a macro.

Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Reviewed-by: Dragan Simic <dsimic@manjaro.org>