libpayload: Fix to properly disable serial console

With coreboot builds with serial console disabled, there is no
CB_TAG_SERIAL entry in coreboot tables. We ended up with
lib_sysinfo.serial == NULL and serial_hardware_is_present == 1.

Change-Id: I9a2fc0b55bf77769f2f2bfbb2b5476bee8083f7d
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: http://review.coreboot.org/5723
Tested-by: build bot (Jenkins)
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>