Tweak the viper control register: - do not enable EISA bus grant until we really attach eisa devices; - on some machines, booting from disk would hang in either "mem0" or "cpu0"   probe. Tinkering eventually pointed out that setting a non-zero non-rachitic   value for the viper's bus error timeout would help.
Tweak the viper control register:
- do not enable EISA bus grant until we really attach eisa devices;
- on some machines, booting from disk would hang in either "mem0" or "cpu0"
  probe. Tinkering eventually pointed out that setting a non-zero non-rachitic
  value for the viper's bus error timeout would help.

Right now, this kluge will only be enabled by default on a known list of
affected machines (which may grow over time...), and device attachment flags
allow users to try the other behaviour if necessary.

Discussed and crafted with mickey@, "it's gross" deraadt@

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars