kernel terminal should initialize fg and bg variables before calling TUNABLE_INT_FETCH

We have two ways to check if kenv variable exists - either we check return
value from TUNABLE_INT_FETCH, or we pre-initialize the variable and check
if this value did change. In terminal_init() it is more convinient to
use pre-initialized variables.

Problem was revealed by older loader.efi, which did not set teken.* variables.

Reported by: tuexen