kernel config: disable virtual terminals

By default systemd starts on all terminals a getty process. But on virtual
terminals an error occurs in getty process. This results the system to hang.
The hikirk board has no keyboard and no monitor. Therefore, we disable all
virtual terminals. This patch disables /dev/tty (not ttyS) and /dev/hvc
virtual terminals.

Signed-off-by: Mario Schuknecht <schuknecht@dresearch-fe.de>
Signed-off-by: Eik Binschek <binschek@dresearch-fe.de>