tools: mtk_image: replace strncpy(d, s, sizeof(d)) with snprintf()

tools: mtk_image: replace strncpy(d, s, sizeof(d)) with snprintf()

Starting from version 8 the GCC, i.e. C compiler, starts complaining about
possible '\0' terminator loss or, as in this case, garbage copy.

In function ‘mtk_image_set_gen_header’,
    inlined from ‘mtk_image_set_header’ at tools/mtk_image.c:733:3:
tools/mtk_image.c:659:2: warning: ‘strncpy’ specified bound 12 equals destination size [-Wstringop-truncation]
  strncpy(hdr->boot.name, bootname, sizeof(hdr->boot.name));
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In function ‘mtk_brom_parse_imagename’,
    inlined from ‘mtk_image_check_params’ at tools/mtk_image.c:388:9:
tools/mtk_image.c:325:5: warning: ‘strncpy’ specified bound 32 equals destination size [-Wstringop-truncation]
     strncpy(lk_name, val, sizeof(lk_name));
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Replace it with snprintf() to tell compiler how much room we have in the
destination buffer for source string.

Fixes: 3b975a147c3c ("tools: MediaTek: add MTK boot header generation to mkimage")
Cc: Ryder Lee <ryder.lee@mediatek.com>
Cc: Weijie Gao <weijie.gao@mediatek.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Weijie Gao <weijie.gao@mediatek.com>