smbios: Define and use enclosure types.

Change-Id: Ib5b92120cbe2ca41c9813e8caeb03161f4d3954c
Signed-off-by: Vladimir Serbinenko <phcoder@gmail.com>
Reviewed-on: http://review.coreboot.org/6786
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Tested-by: build bot (Jenkins)