Expose board-serial and board-product through SMBios

This change exposes the SMBios attributes for the board serial and board product.

This enables for unique identification of Blade nodes, where the chassis serial will be the same as all other blades in the chassis.

We currently use these attributes at Shopify to uniquely identify nodes at boot time, where we can't depend on other attributes such as UUID to exist.

It's a relatively minor change, and expands the existing functionality by exposing additional standard SMBios attributes, which are particularly useful on blade nodes / where there are multiple nodes per chassis.