GENERAL: Move mingw cross-compilation pre-requisites into our own apt repository

[gurcei]

This card came about due to noticing that this repo housed a mingw/ sub-folder containing 1700 files that were used during mingw cross-compilation in linux.

I'd rather these mingw-related files weren't housed within the repo and would prefer them to live in an external apt repo instead.

Ideally, they should one day go into the official Ubuntu/debian repositories, but that might require more polish and refinement in our generated .deb files to meet their approval.

So for now, happy with any first-stab, rough-attempt at .deb file packages housed in our own private repo.

[gurcei]

I've had some success with this recently, so will document usage steps here:

1) Download public key for the repo: sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys F192CFC5C989ADAE

2) Add the repository's address into your apt: sudo add-apt-repository "deb http://gurce.net/ubuntu/ bionic main"

3) Update apt repos with: sudo apt-get update

4) Install the desired package from the repo sudo apt install libpng-mingw-w64 libusb-1.0-0-mingw-w64

You can then run make under linux and it will generate the cross-compiled .exe files for you (for m65.exe, mega65_ftp.exe and bit2core.exe)

[gurcei]

Remaining things to do for this card:

• assess if the newly compiled .exe's work fine in windows (available at http://gurce.net/mega65_files/mingw_exes.zip)
• tweak .travis.yml to make use of our new repo, to install these two pre-reqs from there.