Follows PM discussion with Marcus Comstedt.
Extend changes in .bba file location (made in commit b6a7b60) to ice40 and MSVC cases,
so all cases become compatible with read-only access to git tree.
Only known down-side is inefficiency when building out-of-tree for multiple architectures;
people following that use case should consider using PREGENERATED_BBA_PATH.
It would be nice if there were less copy-paste in MSVC vs. non-MSVC content in family.cmake,
but that would have to be addressed by someone more skilled in Cmake and MSVC.
eba6ea53f8