SeaBIOS: Fix cpp use

No need to pass CPP down to SeaBIOS, it's not
architecture specific and they define their own
variable.

Change-Id: I811aaf3929fa11cc01b7f168ccd310008e21e60c
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/5715
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Tested-by: build bot (Jenkins)
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>