tools/firmware: reproducible seabios build

tools/firmware: reproducible seabios build

The build system of seabios always includes the current time and the
hostname into the resulting binary. To avoid that, it is required to
have a file '.version' in the toplevel directory of seabios-dir-remote.
And it is required to pass EXTRAVERSION= to make because its toplevel
Makefile does not take EXTRAVERSION from environment.

Adjust the code to create a '.version' file with fixed content.
Adjust the code to pass EXTRAVERSION down to make.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Wei Liu <wei.liu2@citrix.com>