Frame/Bitstream writer ignores constraints

[maij]

This is kind of a 2 part issue:

• I would like to be able to generate .mcs files and don't know if I can using prjxray (currently I am happy to just use Vivado tcl to do a one-line conversion between .bit and .mcs)
• To generate the .mcs file correctly for a given flash device, I need to be able to specify certain things in the constraints file, in particular set_property BITSTREAM.CONFIG.SPI_BUSWIDTH 4 [current_design].

I am using the openxc7 toolchain to compile some simple bitstreams, but if I try to convert them to .mcs files using Vivado, it finds that the buswidth is always set to 1. It seems that the .xdc file should be used in either the fasm2frames or xc7frames2bit parts of the flow, but I am not sure where. Is this a current capability of prjxray?