log: provide missing macros

With commit d094a073 ("log: allow for message continuation") we have
defined a special log level and category for message continuation. Let's
have a macro for using these.

If logging is enabled log_cont() will create a continuation log output with
the same logging level and category as the previous message.

If logging is not enabled, log_cont() will print like printf().

Provide macros for logging levels LOG_EMERG, LOG_ALERT, LOG_CRIT.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>