Custom Build Server Enhancements

Feature request

Our Custom Build Server (https://custom.ardupilot.org/) is great, but is lacking a lot of features which would make it very useful. Given that when adding one feature it is often convenient to add several, this is a compendium issue of features which people have mentioned as being potentially useful.

• ability to select a branch to build (instead of "only master")
  • this will require inspecting the selected branch to work out which option are available to present to the user
  • selecting branches from a white-listed github account name list would be a useful follow-up to this
• ability to modify interface based on which vehicle is to be built
• ability to build AP_Periph (linked to above point)
• ability to upload a hwdef.dat (and perhaps other files) to populate the interface. We already allow downloading of the hwdef.dat
• ability to specify default parameter values to be included in the build
• ability to specify some parameters as read-only
• ability to enable assertions
• better dependency tracking / magic update of interface with deps selected (with warning to user that deps have been selected)
• download features.txt from (e.g.) https://firmware.ardupilot.org/Plane/latest/mRoX21-777/features.txt to pre-populate interface when it would be otherwise empty