nanobsd: Fix cust_comconsole search patterns
The current patterns are outdated, and may produce "offifexists", after
e310437971b8 ("For video consoles, only launch a getty if the device
exists.").
Fix the expressions to:
- Disable getty(8) on the virtual syscons(4) or vt(4) terminals (/dev/ttyv*).
- Enable the use of the first serial port as the system console.
Reviewed by: imp
Approved by: emaste (mentor)
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D47586
(cherry picked from commit 0f7b0db5dd07e7c48285db4cac19e15fc9a42622)