smbios: Provide serial number

If the system has a valid "serial#" environment variable set (which boards that
can find it out programatically set automatically), use that as input for the
serial number and UUID fields in the SMBIOS tables.

Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>