Those are cells that are created mainly to handle the various sources a
global network can be driven from other than a user net.
When the flag is set, this means the global network usually driven by
this BEL is in fact driven by something else and so that SB_GB BEL and
matching global network can't be used.
This is also what gets used to set the extra bits during bitstream
generation.
Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
bc9f2da470