Fix booting on systems that use loader(8) (most of them).

r306065/r306067 introduced ofw_parse_bootargs(), setting environment variables
from Open Firmware's /chosen/bootargs property. On systems booting with
loader(8) (meaning, most systems), the initial static kenv is created with no
extra space, causing kern_setenv() to panic. Since these already have the
environment set directly, there is no need to parse bootargs anyway.

Found by: swills