drivers/console: Add static buffer feature to winstream_console

The winstream code itself is reasonably generic, but the console
integration was written to the intel_adsp memory window driver only,
as that was the only consumer.

Add an alternative that allows the winstream to be pointed to by a
linker symbol instead.  The platform layer is responsible for putting
it where it needs to go.

Also includes a 20-byte "descriptor" recodr (with a very large magic
number) in the binary that points to the winstream, for the benefit of
external tooling that needs to find it at runtime.

Signed-off-by: Andy Ross <andyross@google.com>