nextpnr now supports -12k; which replaces the use of -25k and --idcode
together to build bitstreams compatible with -12F devices. Use this.
This also removes the LFEUM-12K and its 5G counterpart; as per Dave Shah
they're currently only theoretical FPGAs.
For most toolchains, these are functionally identical, although ports
tend to work a bit better, being the common case. For Vivado, though,
it is necessary to place them on the port because its timing analyzer
considers input buffer delay.
Fixes#301.
Now environment variable overrides no longer infect the build scripts.
_toolchain.overrides is dropped as probably misguided in the first place.
Fixes#251.
Although useful for debugging, most external tools often complain
about such attributes (with notable exception of Vivado). As such,
it is better to emit Verilog with these attributes into a separate
file such as `design.debug.v` and only emit the attributes that were
explicitly placed by the user to `design.v`.
This still leaves the (*init*) attribute. See #220 for details.
It's not practical to detect tools within the toolchain environment
for various reasons, so just assume the tools are there if the user
says they are.
Before this commit, the tools would be searched outside the toolchain
environment, which of course would always fail for Vivado, ISE, etc.
Right now an array is expected in any _opts overrides, and if it is
actually a string (because it is passed via an environment variable,
usually), awkwardness results as each character is joined with ` `.
Fixes#130.
