[smbios] Provide SMBIOS version number via smbios_version()

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[smbios] Mangle UUIDs for SMBIOS version 2.6 and newer

iPXE treats UUIDs as being in network byte order (big-endian).  The
SMBIOS specification version 2.6 states that UUIDs are stored with
little-endian values in the first three fields; earlier versions did
not specify an endianness.  This results in some inconsistency between
the BIOS, vendor PXE, iPXE, and operating system interpretations of
the SMBIOS UUID.

dmidecode assumes that the byte order is little-endian if and only if
the SMBIOS version is 2.6 or higher.  Choose to match this behaviour.

Reported-by: Matthew Helton <mwhelton@gmail.com>
Reported-by: Alexandru Bordei <alexandru.bordei@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>