stand: Parse BIOS revision from SMBIOS

Add a smbios.bios.revision kenv, which contains the system BIOS revision
as defined in SMBIOS specification, section 3.3.1.
Since the revision is stored in two separate byte fields,
the smbios_setenv helper can't be used.
Read and construct the kenv manually instead.

Approved by: mw(mentor)
Sponsored by: Stormshield
Obtained from: Semihalf
Differential Revision: https://reviews.freebsd.org/D36413

(cherry picked from commit 66c73af7ea59382ce5ca7cfb2eedb0491790276c)