Drop-down list of board choices for firmware loading

The current "auto-detect and fallback to wizard" method of selecting the right firmware for a board is not working well. There are several problems with it:
- the auto-detection can't cope with boards where there are multiple possible firmwares (eg. px4-v3 and fmuv3)
- the wizard asks questions like "is this is pixhawk", but what is the answer for a cube? Some users have answered yes and ended up with incorrect compass orientations etc

I think a better solution would be a drop-down list of all possible boards, with the auto-detection code just giving the default value. This means the user can immediately see all the possible selections.
This is particularly important now we're adding separate firmwares for Pixhawk4, PH4-mini, CUAVv5, CubeBlack etc. One of the best ways of users starting to become aware of what firmware they should use would be this drop-down list.

ping @rmackay9

Start by locating Mission Planner's current board auto-detection and fallback wizard, then inspect the firmware-loading selection UI. Done means users can choose from all supported boards in a drop-down, with auto-detection supplying the default selection.

Written by the indexing model from the issue text.