Disable boot-time memory test on i386 be default.

With the current 24G memory limit for GENERIC, the boot time test
causes quite visible delay, amplified by the default
debug.late_console = 0.

The comment text is copied from the same setting explanation for
amd64.

Suggested by: bde
Discussed with: emaste
Sponsored by: The FreeBSD Foundation
MFC after: 2 months