core/say: allow to use json in boot logger

For some reason in commit 09832455 we've disabled
to use json format in boot time logger. There is
no reason to do so.

Only syslog output format is predefined and must not
be changed, in turn json format is just a decoration
over output stream so we can use it whenever requested.

Part-of #5121

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>