servo: For all commands that read stdout, decode

I found this in a moblab repair log:
```
Traceback (most recent call last):
  File "/usr/local/autotest/client/common_lib/hosts/repair.py", line 668, in _repair_host
    self.repair(host)
  File "/usr/local/autotest/server/hosts/cros_firmware.py", line 191, in repair
    self._run_repair(host, build)
  File "/usr/local/autotest/server/hosts/cros_firmware.py", line 228, in _run_repair
    if host._servo_host.validate_image_usbkey() != build:
  File "/usr/local/autotest/server/hosts/servo_host.py", line 518, in validate_image_usbkey
    return self.get_image_name_from_usbkey(usb_dev)
  File "/usr/local/autotest/server/hosts/servo_host.py", line 341, in get_image_name_from_usbkey
    if not re.search(r'RELEASE_TRACK=.*test', release_content):
  File "/usr/local/lib/python3.7/re.py", line 185, in search
    return _compile(pattern, flags).search(string)
TypeError: cannot use a string pattern on a bytes-like object
```

To fix this, check all stdout strings to make sure they are not bytes.

BUG=b:217780680
TEST=None. CP from crrev/c/3671361

Change-Id: I7c9b59f48dc48b018878d8a0a67bce55a0662a96
1 file changed