This patch adapts the boot tracer to the new type of the
print_line callback.

It still relays entries it doesn't support to default output
functions.
Signed-off-by: NFrederic Weisbecker <fweisbec@gmail.com>
Acked-by: NPekka Paalanen <pq@iki.fi>
Signed-off-by: NIngo Molnar <mingo@elte.hu>