Use fast memset in SMM mode, too

Use fast memset in SMM mode, too

... and always include IP checksumming in romstage.
It's generally useful and our upcoming port needs it.

Change-Id: I248402d96a23e58354744e053b9d5cca6b74ad3a
Signed-off-by: Stefan Reinauer <reinauer@google.com>
Reviewed-on: http://review.coreboot.org/827
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>