[linux] Provide access to SMBIOS via /dev/mem

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[dhcpv6] Use DUID-UUID form of client DUID

Use the system UUID to generate the client DUID-UUID as per RFC 6355.

Signed-off-by: Michael Brown <mcb30@ipxe.org>