Two user-visible changes were made:
* `-DUSE_RUST` is replaced with `-DBUILD_RUST`, by analogy with
`-DBUILD_PYTHON`
* `-DCOVERAGE` was removed as it doesn't work with either modern GCC
or Clang
Primarily, this commit makes both of them use the `BBAsm` functions
to build and compile `.bba` files.
In addition, Himbaechel targets are now aligned with the rest in
how they are configured: instead of having all uarches enabled with
all of the devices disabled (the opposite of the rest of nextpnr),
uarches must be enabled explicitly but they come with all devices
enabled (except for Xilinx, which does not have a list of devices).
