Add kernel environment variables under smbios.system for the following
SMBIOS Type 1 fields:
smbios.system.sku - SKU Number (SMBIOS 2.4 and above)
smbios.system.family - Family (SMBIOS 2.4 and above)

Add kernel environment variables under smbios.planar for the following
SMBIOS Type 2 fields:
smbios.planar.tag - Asset Tag
smbios.planar.location - Location in Chassis

Reviewed by: jhb, grembo
Approved by: sjg (mentor)
MFC after: 2 weeks
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D7453