flashy-f5510f3

Restore fw_printenv command

Summary:
D60115096 introduced a potential bug whereby `fw_printenv` check is expecintg stdout from the previous command, which is now replaced by an `ls` command

Restore the previous command and clearly differentiate between the two errors

Test Plan:
Unit tests
```
$ ./tools/flashy/scripts/run_unit_tests.sh
...
...
...
142/142 unit tests passed
```

Reviewed By: kawmarco

Differential Revision: D60234128

fbshipit-source-id: 50ebd779d47082ca8487bbf53bb68d5db292c883