empty design causing icetime error: No path found!

[gojimmypi]

The current Makefile calls icetime at the same time as nextpnr. The problem is that icetime generates an error: No path found! (despite actually creating the rpt file). This is apparently a design error, not a file system error. See also YosysHQ/icestorm#205 and YosysHQ/icestorm#45.

The first time running make results in this error:

[ ... snip ... ]
icetime -d hx8k -c 50 otl-modem.asc
// Reading input .asc file..
// Reading 8k chipdb file..
// Creating timing netlist..
// Timing estimate: 0.00 ns (inf MHz)
No path found!
Makefile:38: recipe for target 'otl-modem.asc' failed
make: *** [otl-modem.asc] Error 1

As the otl-modem.rpt and otl-modem.asc files are still created despite the error message, running make again appears to be otherwise successful.

Further details can be found in YosysHQ/icestorm#204

See PR #1 for suggested resolution.