imx-boot: Fix the symbolic link filename to imx-boot
Fix the error:
| File "/media/daiane/TRIFORCE/yocto/master/sources/poky/scripts/lib/wic/filemap.py", line 132, in _open_image_file
| % (self._image_path, err))
| wic.filemap.Error: cannot open image file '/media/daiane/TRIFORCE/yocto/master/build/tmp/deploy/images/imx8mmevk/u-boot.imx': [Errno 2] No such file or directory: '/media/daiane/TRIFORCE/yocto/master/build/tmp/deploy/images/imx8mmevk/u-boot.imx'
