Set to debug mode if onie_verbose is set

Per Curt's point that ONIE actually has a verbose parameter that it can set as well.

Reviewers: @jnealtowns @nshopik 
onie_verbose is not a U-boot env variable.  It is a shell env variable that the ONIE image discovery/executive program sets when running your installer.  So you would want something like:

```bash
[ "$onie_verbose" = "y" ] && install_debug=1
```