Adapt the build scripts for the Qemu platform
Created by: jmarinho
Set of changes:
- allow StMM platform file to be set per-board;
- parametrize qemu bin;
- make ebbr.config optional (selectable via the ebbrconfig target);
- enable TF-A tbbr secure boot in qemu;
- allow for a NVME drive;
- add qemu debug targets.