seabios: update to 1.8.1 stable release

Carries two bugfixes and support for multiple pci root buses.

git shortlog rel-1.8.0..rel-1.8.1
=================================

Ameya Palande (1):
      x86: add barrier to read{b,w,l} and write{b,w,l} functions

Kevin O'Connor (1):
      smp: Fix smp race introduced in 0673b787

Marcel Apfelbaum (2):
      fw/pci: scan all buses if extraroots romfile is present
      fw/pci: map memory and IO regions for multiple pci root buses

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>