debug: allow debug_early to work really early again

an effect of commit 9bbbe9dc is that no output is possible
before prio 'console' initializers have been run.
This change allows to have at least one API available
really early (from boot code and premain).
Document the requirements for the early console class
regarding the write() method.

Signed-off-by: Claudio Fontana <claudio.fontana@huawei.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>