sunxi: Ensure that 'mksunxiboot' tool produces deterministic output

Currently some uninitialized padding bytes are written to the output
file, as can be confirmed with valgrind:

$ valgrind tools/mksunxiboot spl/u-boot-spl.bin spl/sunxi-spl.bin

==5581== Syscall param write(buf) points to uninitialised byte(s)
==5581==    at 0x4F0F940: __write_nocancel (in /lib64/libc-2.20.so)
==5581==    by 0x400839: main (in /tmp/u-boot/tools/mksunxiboot)
==5581==  Address 0xffeff5d3c is on thread 1's stack
==5581==  in frame #1, created by main (???)

This patch fixes the problem by clearing the whole structure instead
of just a portion of it.
Signed-off-by: NSiarhei Siamashka <siarhei.siamashka@gmail.com>
Acked-by: NHans de Goede <hdegoede@redhat.com>
Signed-off-by: NHans de Goede <hdegoede@redhat.com>

tools/mksunxiboot.c

@@ -99,7 +99,7 @@ int main(int argc, char *argv[])
 		return EXIT_FAILURE;
 	}
 
-	memset(img.pad, 0, BLOCK_SIZE);
+	memset(&img, 0, sizeof(img));
 
 	/* get input file size */
 	file_size = lseek(fd_in, 0, SEEK_END);