Add option to bit2fasm to skip emitting empty CLE sites

[litghost]

daveshah: UltraScale+ devices have all unused LUTs set to output 1 by default, to save power
daveshah: this results in loads of extra set bits in the FASM
daveshah: it might be nice to have a bit2fasm option that skips these

[litghost]

I would propose we continue to emit the bits for unused LUT's within a used CLE, but totally empty CLE's we can safely drop. There would need to be a corresponding change to fasm2bit to emit unused CLE's with the default LUT configuration.

[daveshah1]

SGTM - so long as explicitly setting LUT init to 0, even if the site is otherwise empty, doesn't still set the default 1s as this may be a genuine situation for pseudo-ground drivers - UltraScale+ doesn't have xc7-style ground nodes.

[litghost]

SGTM - so long as explicitly setting LUT init to 0, even if the site is otherwise empty, doesn't still set the default 1s as this may be a genuine situation for pseudo-ground drivers - UltraScale+ doesn't have xc7-style ground nodes.

The logic would be "if there is a FASM feature using the CLE, do not emit defaults", so setting the ALUT.INIT = 64'b0 would work as expected.