Make the ABI check script a bit more reusable

This is a bit of progress on #403 (closed):

• the script now builds/checks out a single element (newly introduced abi/desktop-abi-image.bst) which contains everything we need, instead of taking stuff from both desktop-platform-image.bst and desktop-image.bst;
• the script now takes the element to build/checkout as an argument;

This should help our friends at GNOME: https://gitlab.gnome.org/GNOME/gnome-build-meta/merge_requests/74