imx-mcore-demos: Fix 7D

For 7D there are no .elf files and there is a build break:
```
| install: cannot stat '.../imx7d-sabresd-m4-freertos-1.0.1/*.elf': No such file or directory
```

Fixes #2056.

Signed-off-by: Tom Hochstein <tom.hochstein@nxp.com>