tools: support SeaBIOS. Use by default when upstream qemu is configured.

tools: support SeaBIOS. Use by default when upstream qemu is configured.

The SeaBIOS integration here is only semi-complete and is targetted at
developers and very early adopters who can be expected to cope with
some rough edges. In particular the user must clone, patch as
necessary and compile SeaBIOS themselves since this patchset does not
cover any of that (in the same way we currently do not integrate
upstream qemu clone+build). Include a big comment to that effect next
to the Config.mk option.

Many of the bios_config callback functions are not yet used by
SeaBIOS.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson.citrix.com>
Committed-by: Ian Jackson <ian.jackson.citrix.com>