x86: early_serial: Suppress output attempts prior to init

Until now, any attempts to call printk prior to early serial init has
caused page faults due to the device not being mapped yet. Add static
variable to track the pre-init status, and instead of page faulting
just suppress the characters and log a warning right after init to
give an indication that output characters have been lost.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>