Add option to enable buster-backports and install freedombox package from backports

Backports are used only during build time and not added into the image itself since the freedombox package takes care of it.

Planning to use the --enable-backports option in all stable images built by the CI from now on (maybe till Bullseye is released)

Testing

I built an amd64 image, converted it into VDI and verified that the latest freedombox package from backports is installed in it.

sudo python3 -m freedommaker amd64 --distribution stable --build-in-ram --skip-compression --enable-backports
cd build
VBoxManage convertdd freedombox-stable-free_2020-04-05_all-amd64.img freedombox-stable-free_2020-04-05_all-amd64.vdi --format VDI

Also added a unit test.