rc.d/hostid: Skip warning on systems w/o smbios

The first time a FreeBSD system boots, it obtains a hostuuid and hostid
from the smbios.system.uuid kernel environment variable. If this value
is found to be invalid, a warning is printed and the boot pauses for
two seconds to give the user a chance to read it.

If the FreeBSD kernel is launched directly in a virtual machine rather
than via the FreeBSD boot loader, the smbios.system.uuid environment
variable might not be set; in this case, there's no need to alert the
user and delay the boot process since the lack of a "hardware" uuid is
entirely expected.

Distinguish between the cases of "invalid UUID" and "no UUID", warning
and delaying the boot only in the former case. In both cases we still
generate a random UUID in software.

Reviewed by: delphij
Sponsored by: https://www.patreon.com/cperciva
Differential Revision: https://reviews.freebsd.org/D36185