MP 1.3.49 doesn't recognize a Ph2.1 board for fw update

#### Issue details
I just installed 1.3.49 release and have the problem that I can't update my Pixhawk 2.1 anymore. It simply won't detect and recognize the board at the beginning of the update.
Otherwise MP works well, I can connect through USB, read and write parameters, HUD and all displays work seamlessly. So it seems not to be an USB driver issue.

I then downgraded back to the 1.3.48 version and update worked as usual.
I then re-updated to the latest release 1.3.49 and the detection of the Ph failed again. Since I frequently updated Arducopter during the AC beta testing with 1.3.48 and prior betas w/o problems I assume a defect in the current release of MP.

Setup: Win32bit laptop with the latest Win10 upgrades installed.

#### Version
MP 1.3.49 official release

#### Platform
[ ] All
[ ] AntennaTracker
[ X ] Copter
[ ] Plane
[ ] Rover
[ ] Sub

#### Airframe type
_What type of airframe (flying wing, glider, hex, Y6, octa etc)_
Quad

#### Hardware type
_What autopilot hardware was used? (pixhawk, pixracer, PX4FMU etc)_
Pixhawk 2.1 (cube)

#### Logs
_Please provide a link to any relevant logs that show the issue_

Reproduce the firmware-update detection failure with Mission Planner 1.3.49 and a Pixhawk 2.1 on the described Windows setup, then compare it with 1.3.48. Trace the update startup path to identify why the board is not recognized; done means the Pixhawk 2.1 is detected and can be updated on 1.3.49.

Written by the indexing model from the issue text.