While it served a purpose (granting the ability to build `.bba` files
separately from the rest of nextpnr), it made things excessively
convoluted, especially around paths.
This commit removes the ability to pre-generate chip databases. As far
as I know, I was the primary user of that feature. It can be added back
if there is demand for it.
In exchange the per-family `CMakeLists.txt` files are now much easier
to understand.
I believe Prjtrellis has CMAKE define pytrellis as a shared module, not library.
On MacOS, this makes a difference and NextPNR expected ``.dylib``
instead of ``.so``. Things still work on Linux.
`if(NOT DEFINED)` is not appropriate since a variable that contains
`-NOTFOUND` still counts as `DEFINED`. This can cause issues if
configuration fails, writes `-NOTFOUND` to the cache, and is then
restarted.
