smbios: Don't fill out firmware version on ChromeOS

In ChromeOS we potentially have different payloads with
different versions. Since the user land tools get information
on which one of them is loaded, leave the string in smbios
empty so we can fill it out in the payload.
Also fill out system version number and serial number with
some constant values.

Change-Id: Id1fed5a54b511c730975fa83347452f1274b8504
Signed-off-by: Stefan Reinauer <reinauer@google.com>
Reviewed-on: http://review.coreboot.org/867
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>
Tested-by: build bot (Jenkins)